In vivo effect of toxaphene on calcium pump activity in rat brain P2fraction was studied. Male Sprague‐Dawley rats (200–250 g) were dosed with toxaphene at 0, 25, 50, and 100 mg/kg · d for 3 d and sacrificed 24 h after last dose. Ca2+‐ATPase activity and45Ca2+uptake were determined in brain P2fraction. Toxaphene decreased both Ca2+‐ATPase activity and45Ca2+uptake, and the reduction was dose‐dependent. Both substrate and Ca2+activation kinetics of Ca2+‐ATPase indicated noncompetitive type of inhibition, as evidenced by decreased catalytic velocity but not enzyme‐substrate affinity. The decreased Ca2+‐ATPase activity and45Ca2+uptake were restored to normal level by exogenously added calmodulin, which increased both velocity and affinity. The inhibition of Ca2+‐ATPase activity and45Ca2+uptake and restoration by calmodulin suggests that toxaphene may impair active calcium transport mechanisms by decreasing levels of calmodulin.