Activation of serotonin neurons in the raphe nucleus has been associated with waiting for delayed rewards in rats, but whether this reflects a causal relationship was not known. Here, the authors used a navigation task in which rats had to remain in a fixed posture at a reward site to receive a food or water reward. Inhibition of serotonin neuron activity in the dorsal raphe nucleus caused rats to fail to wait for a reward if the delay was long, whereas it had no effect under conditions in which the delay was short. The inhibition did not affect cognitive or motor functions. These findings suggest that activation of serotonin neurons is required for waiting for delayed rewards.