Twelve male volunteers smoked a marihuana cigarette prepared to deliver 0 or 25 µ/kg tetrahydrocannabinol (THC) 50 min after ingesting a capsule containing either placebo or 150 mgl70 kg sodium secobarbital. Drugs were administered in a double-blind manner, and all treatments were assigned to each subject in a randomized complete block design. Objective and subjective tests designed to measure mental and motor performance indicated that marihuana impaired stability, hand-eye coordination, and mental performance. Secobarbital affected motor performance, manual coordination, and mental performance. In combination, marihuana and secobarbital had an additive effect on subjective responses and impairment in certain psychomotor performance tests.