(-)Deprenyl is an irreversible inhibitor of monoaminoxidase-B (MAO-B) at concentration 10(-6)M and of both MAO-A and MAO-B at concentration 10(-5)M. In this study, the effect of different concentrations (10(-7)-10(-4)M) of (-)deprenyl on the activity of acetylcholinesterase (AChE), Na(+),K(+)-ATPase and Mg(2+)-ATPase was investigated in homogenates of adult rat whole brain and in pure enzymes. Drug preincubation period with the homogenates or pure enzymes was 1 and 3h. Brain AChE activity was decreased by 30-39% (P<0.01) when exposed to 10(-4)M (-)deprenyl, by 22-25% (P<0.01) when exposed to 10(-5)M of the drug, and by 18-20% (P<0.01) after preincubation with a concentration of the drug 10(-6)M. Brain Na(+),K(+)-ATPase was stimulated by 46-162% (P<0.001) when the homogenate was preincubated with 10(-4)M (-)deprenyl and by 36-104% (P<0.001) for preincubation with drug concentration 10(-6)M. No effect was observed on the activity of brain Mg(2+)-ATPase, pure AChE or pure Na(+),K(+)-ATPase when exposed to the above concentrations of the drug. We conclude that (-)deprenyl is an indirect AChE inhibitor and Na(+),K(+)-ATPase stimulator in the rat brain (in vitro).