One long-term goal of aging research is to find drugs that can delay aging and the onset of age-associated diseases. With this in mind, we screened 88,000 chemicals for the ability to increase the lifespan of Caenorhabditis elegans nematodes. We found that mianserin, a serotonin receptor antagonist used as an antidepressant in humans, can increase C. elegans lifespan when given only during adulthood. This effect is reduced or abolished by mutations that affect serotonin synthesis or serotonin reuptake at synapses. It also requires a serotonin receptor and an octopamine receptor, both of which are inhibited by the drug. Mianserin has no effect on the lifespan of animals with increased longevity due to dietary restriction or with a mutation that reduces food intake, indicating that the drug extends lifespan via mechanisms linked to dietary restriction. These studies indicate that lifespan can be increased by inhibiting certain kinds of neurotransmission previously implicated in food sensing, possibly by mimicking a physiological state associated with dietary restriction.