Recent work with mice has revealed that the antidepressant Prozac spurs growth of new neurons in the brain. Prozac, or fluoxetine, is thought to ease depression by raising the level of the neurotransmitter serotonin in the brain. But now researchers have learned that the drug also causes more neurons to form than normally would. In mice, blocking this growth nullifies the drug's effects on behavior, suggesting that neuron formation may be part of the mechanism that alleviates depression.