When an animal learns that a conditioned stimulus (CS) predicts a reward, midbrain dopaminergic neurons become excited by the reward-predicting CS. The neural circuits that generate this CS-elicited excitation are unknown. We recorded the activity of identified dopaminergic neurons in the ventral tegmental area of mice in a classical conditioning task, in which odors or tones predicted different probabilities of reward or punishment. During the task, we reversibly inactivated a region of the basal forebrain with a local infusion of muscimol, a GABAA receptor agonist. Before basal forebrain inactivation, dopaminergic neurons were phasically excited by a reward-predicting CS. Basal forebrain inactivation selectively eliminated this excitation. These results suggest that information about reward-associated cues reaches dopaminergic neurons via the basal forebrain.