Abstract Effect of toxaphene on Ca 2+-ATPase activity in rat brain synaptosomes was studied in vitro and in vivo. Ca 2+-ATPase in calmodulin-depleted synaptosomes was inhibited in vitro to a maximum of about 50% at 150 μ m toxaphene. Substrate activation kinetics of Ca 2+-ATPase in synaptosomes revealed that toxaphene inhibited the enzyme activity noncompetetively by decreasing V max values, without affecting the enzyme-substrate affinity. Toxaphene inhibited the calmodulin activated Ca 2+-ATPase activity in a concentration-dependent manner with an IC50 of 10 μ m, a concentration at which no significant effect was observed on basal enzyme activity. Nuclear and P 2 fraction (synaptosomes) calmodulin levels were reduced significantly in toxaphene-treated rats. The synaptosomal Ca 2+-ATPase was also reduced to about 45% in toxaphene-treated rats and the activity was restored to normal levels by the exogenously added calmodulin. These results suggest that toxaphene may cause synaptic dysfunction by interfering with calmodulin and its regulation of neuronal calcium.