According to St. Thomas, the very first principle of practical reasoning in general is: The good is to be done and pursued; the bad is to be avoided (S.t., 1–2, q. 94, a. 2). This is a directive for action, not a description of good and evil. “Good” here means not only what is morally good but whatever can be understood as intelligibly worthwhile, while “bad” refers to whatever can be understood as a privation of intelligible goods. Thomas’ formulation—“Good is to be done and pursued” rather than “Do good!”—suggests that he thinks this principle extends to and governs all coherent practical thinking.
In scholastic natural-law theory, the first principle of practical reasoning was misunderstood, formulated not as St. Thomas did but as a most general moral imperative: Do good and avoid evil. However, the first principle is not a moral norm; it governs morally good and morally bad thinking alike. Moreover, an imperative to do good and avoid evil would not be self-evident. Confronted with this command, one could reasonably ask: Why?
This whole line of argument I find simply bizarre. Aquinas does not say that the first principle of practical reason is "Good is to be done and evil is to be avoided." The first principle of practical reason, according to Aquinas, is "Good is what all desire," [bonum est quod omnia appetunt] i.e., by its very nature as good, good has the nature of an end of practical action and ends of practical action have in some way the nature of good. So where does "Good is to be done and pursued and evil is to be avoided" come in? Aquinas tells us (quite explicitly) that it is the first precept of law [Hoc est ergo primum praeceptum legis, quod bonum est faciendum et prosequendum, et malum vitandum]. Obviously the two are closely related, so in most cases it's not a big deal if one shifts back and forth between them or doesn't distinguish them very carefully, but Grisez's whole argument depends on sharply distinguishing the first principles of practical reason from moral imperatives. And he puts the first moral imperative on the wrong side of the line. The scholastic manualists, not Grisez, have the right interpretation of Aquinas: "Good is to be done and pursued, and evil is to be avoided," while not a grammatical imperative, has as a precept imperative force, and one can easily and naturally state it as "Do and pursue good and avoid evil!"
Grisez is right that the first principle of practical reason extends to all practical reasoning, even kinds that we don't normally consider moral, because without it no practical reasoning would be possible. But the first principle of practical reasoning as Aquinas understands in itself it tells us, effectively, that no sense can be made of denying that good is an end, or that what you take as an end you do so because you regard it as in some way good. When we regard it as having the character of law and imperative, the imperative is that good is to be done and evil avoided. Likewise, the distinction Grisez is making between moral norms and non-moral norms simply doesn't exist in Aquinas, unless you treat it as a distinction between natural laws that prescribe virtues and natural laws that don't. But this does not change the fact that in some sense all the precepts of natural law are moral precepts, precisely because they are precepts with regard to what is good.
Thus I find Grisez's last two sentences in the passage above incomprehensible when we are understanding the terms as Aquinas (and the scholastic manualists) understands it. As a precept the command "Do good and avoid evil" admits of justification: it can be justified by the self-evident links, posited by the first principles of practical reason, between what is good and what one takes as an end. 'Self-evident' is a term that properly applies to principles, not precepts, at least considered as precepts. Certain precepts of reason are derived from self-evident principles, since they outline the actions you need to do in order not to act irrationally in light of those self-evident principles. But that's the only sense in which an imperative or precept can be considered self-evident. If someone, confronted with the command "Do good and avoid evil," responds with "Why?" rather than "OK, but what's good and what's evil?" they are either (1) being as irrational as someone who thinks that they can still believe two things when they have rigorously proven to their own satisfaction that they can't both be true or (2) not understanding the language you are using or (3) merely trying to determine the precise character of the principle from which this precept derives.
Likewise, all practical reasoning, whether moral or immoral, depends on both the first principle of practical reasoning and the first precept of the natural law, as Grisez suggests, but this is only in the sense that all thinking, whether consistent or inconsistent, depends on the principle of noncontradiction. Grisez himself explicitly recognizes this feature of the principle of noncontradiction, and explicitly follows Aquinas in treating the first principle of practical reason as analogous to the principle of noncontradiction, but his argument is very difficult to reconcile with this line of thought. And I think it's largely for the reason given above, namely, that he is mostly following Aquinas but both draws the line between first principle and first precept in an entirely different place and imposes on the whole a moral/nonmoral distinction that is very foreign to Aquinas. The result is confusion.
These problems, of course, are closely related to standard objections to Grisez's New Natural Law Theory, which are summarized nicely in the first part of Tollefsen's article on New Natural Law.
UPDATE: As Toomas Vooglaid notes in the comments, some of the things said here are seriously misleading as stated. See my reply to Tom for a clarification.