We studied the effects of amitriptyline on diurnal variations of oxidative-reduction enzyme activities (succinate dehydrogenase, lactate dehydrogenase, and NADPH diaphorase) in rat peripheral blood lymphocytes during experimental desynchronosis. Desynchronosis was induced by constant light exposure for 14 days and manifested in a morning shift of maximum lactate dehydrogenase activity and elevated morning NADPH diaphorase activity. Amitriptyline normalized morning lactate dehydrogenase activity and diurnal variations in NADPH diaphorase activity.