To show it is sometimes rational to cooperate in the Prisoner's Dilemma, David Gauthier has claimed that if it is rational to form an intention then it is sometimes rational act on it. However, the Paradox of Deterrence and the Toxin Puzzle seem to put this general type of claim into doubt. For even if it is rational to form a deterrent intention, it is not rational act on it (if it is not successful); and even if it is rational (...) to form an intention to drink a toxin, it is not rational to act on it (come the time for drinking). This article employs an extended version of Michael Bratman's theory of intention to show how to argue systematically that it can be rational to act on rationally formed cooperative intentions, while not being committed to the rationality of apocalyptic retaliation, or pointless toxin drinking. (shrink)