The nocturnal activity of a primate was used as an evaluation criterium for a stimulating substance: adrafinil (CRL 40028). Ten rhesus monkeys were placed in a controlled environment and their activity was measured, in relative time, using an ultra-sound system. The animals repeatedly received 60, 90 and 120 mg X kg-1 adrafinil per os. Globally, the dose of 60 mg X kg-1 doubled the animals' nocturnal activity whereas 90 and 120 mg X kg-1 increased it fourfold, the activity level becoming practically identical to diurnal activity. The effects of 60 mg X kg-1 were only significant after the second treatment whereas doses of 90 and 120 mg X kg-1 were already significantly efficient after the first administration. A stimulating effect persisted approximately 36 hrs after the second treatment with 90 or 120 mg X kg-1. No sedative effect of recovery was observed during the posttreatment phase.