Triazolam can shift the phase of circadian rhythms in hamsters recorded in constant light or dark. This effect is apparently mediated by physical activity stimulated by the drug. We examined whether triazolam can shift the phase of circadian rhythms in a diurnal primate, the squirrel monkey, that is sedated by triazolam. Single injections of triazolam at 0.15-0.20 mg/kg induced phase advances or delays of monkey activity rhythms recorded in constant light. The phase-response curve is similar to that obtained for hamsters. Behavioral activation is evidently not necessary for a phase-shifting action of triazolam in this primate. A companion study in which triazolam was administered to squirrel monkeys in the dark in a light-dark cycle reentrainment paradigm failed to find evidence for a phase shifting action of triazolam. Shifts induced by triazolam in monkeys recorded in constant light may thus reflect changes in light exposure as a consequence of sedation or altered retinal processing of light.