Physical determinism is a metaphysical thesis about the natural world whose consequences for freedom and moral responsibility have been widely discussed. In this paper, I articulate a different form of determinism, motivational determinism, which claims that all intentional action is causally determined by the prior motivational state of the agent. Motivational determinism was defended in a simple form by Hume, but has been neglected in recent philosophical literature. I show that there are important reasons that support the plausibility of MD. (...) I then compare MD with libertarian and physical deterministic theories of agency and note that MD has various attractive features. I show that MD is much more threatening to moral responsibility than physical determinism. I conclude that MD is an interesting theory of human agency that deserves wide attention. (shrink)