The in vivo and in vitro effects of the pesticide endosulfan on the cholinesterase (ChE) activity were investigated in rats. ChE activity decreased in dams and in male pups within 65 days corresponding to 35% and 32% of inhibition respectively in the higher endosulfan dose (1.5 mg/kg). In vitro, the enzyme activity was found to be inhibited in a concentration dependent manner. The results suggest that endosulfan is able to inhibit the ChE activity and to cross the placental barrier and/or to be eliminated through milk affecting the enzyme activity in male rat pups.