Calmodulin‐dependent (CaM‐dependent) and nondependent Ca²⁺ pump activity was determined in the brain P₂ fraction of Sprague‐Dawley rats treated in vivo with 10 ppm dieldrin for 60 d. After 40 d of dieldrin intoxication, daily food consumption and body weight gain were found to be affected. Dieldrin inhibited both CaM‐dependent and ‐independent Ca²⁺‐ATPase activity and the enzyme activity decreased to 36% (CaM‐dependent)—and 18% (basal) respectively after 60 d of intoxication. Reduction in brain CaM levels and a subsequent inhibition of Ca²⁺‐ATPase activity suggests that chronic intoxication of dieldrin at very low doses alters calmodulin and its regulation of Ca²⁺‐ATPase activity.