1 Residual effects in man of methaqualone hydrochloride (400 mg) were studied by adaptive tracking and by reaction time. Performance was measured at 10 h, 13 h, 16 h, 19 h and 34 h after the overnight ingestion of the drug. There was no evidence of impaired performance on adaptive tracking from 10 h to 19 h, but enhanced performance (P = 0.001) was observed 34 h after ingestion. With reaction time an increase (P = 0.01) was observed 10 h and a decrease (P = 0.05) was observed 19 h after ingestion. 2 Effects in the monkey (Macaca mulatta) of methaqualone (20 and 30 mg/kg body weight) were studied by a delayed matching task in which total response time was measured. No consistent effects on matching behaviour or on total response time were observed 2 h after intraperitoneal injection. 3 The studies suggest that methaqualone hydrochloride may be a valuable hypnotic for occasional use by persons involved in skilled activity.