Researchers have hypothesized that dopamine mediates the reinforcing effects of stimulant drugs, including nicotine. Three experiments tested whether manipulating dopamine would alter human smoking behavior. Experiments used double-blind, repeated measures designs. In Experiment 1, 4 participants were given haloperidol (a dopamine antagonist; placebo, 0.5, and 1.0 mg) on 3 occasions. The smoking rate was faster in the 1.0 mg versus the placebo condition. In Experiment 2, 12 participants were given haloperidol (2.0 mg) and placebo on 2 occasions. The intercigarette interval was shorter at the expected time of peak drug concentration. In Experiment 3, 5 participants were given bromocriptine (a dopamine agonist, 2.5 mg) and placebo on 2 occasions. The smoking rate was significantly slower with bromocriptine. These results suggest that blockade of D2 receptors increases smoking whereas their stimulation decreases smoking.