Nitrazepam (0.5-10 mg/kg, IP) dose dependently induced behavioral sleep in day-old chicks. Fluoxetine (0.1-1 mg/kg) did not produce sleep in the young birds, but the 5-HT reuptake inhibitor (0.1 and 0.5 mg/kg) potentiated nitrazepam (0.5 and 1.0 mg/kg)-induced hypnosis. Doses (0.5 and 1 mg/kg) of the benzodiazepine that did not produce sleep in any or all the chicks, when administered alone, induced sleep in some or all the chicks in the presence of fluoxetine (0.1 and 0.5 mg/kg). Ketanserin (0.5 and 2.5 mg/kg) effectively antagonized the effect of fluoxetine on nitrazepam-induced behavioral sleep. These results suggest that enhancement of 5-HT level by fluoxetine may be the mechanism involved in the potentiation of nitrazepam-induced sleep in the young chicks.