Stimulants, in particular the amphetamines, have been studied as countermeasures to fatigue induced by circadian desynchronosis and extended flight operations. To make recommendations concerning the use of dextromethamphetamine for operational tasks, its chronopharmacokinetic and chronopharmacodynamic profiles and influence on circadian rhythms as a countermeasure to performance deficits and fatigue were studied. Ten male volunteers, divided into two groups of five each, were given 30 mg/70 kg of oral dextromethamphetamine during two test sessions one week apart and were evaluated with cognitive (dichotic listening, pattern recognition, and compensatory tracking), subjective (fatigue scale), and physiologic (blood pressure) testing. Session order was counterbalanced with dextromethamphetamine administration at either 8:40 AM or 8:40 PM during session one and a crossover to the other time during session two. Subjective and cognitive testing was begun 1.5 hours before dextromethamphetamine administration and continued every half hour until 12.5 hours after administration. Blood pressure was measured immediately before behavioral testing. Serum and urine were collected at regular intervals for gas chromatography/mass spectrometer analysis of methamphetamine and one of its metabolites, amphetamine. No differences were found in the day-versus-night pharmacokinetic profile of dextromethamphetamine. Cognitive performance and subjective fatigue improved after daytime administration of dextromethamphetamine in comparison to performance before drug administration. This effect was suppressed during the circadian trough, which occurred approximately 8 hours into the night sessions (4:30 AM). No correlations were seen between serum concentration of methamphetamine and measured behavioral parameters.