The present report describes the activity of aspartate aminopeptidase, in 9 brain areas and in the pituitary gland, during the estrous cycle and several stages of pregnancy of the rat. The quantitation of the enzyme activity was performed by measuring the rate of hydrolysis of the chromogenic substrate Asp-beta-naphthylamide. Aspartate aminopeptidase activity, in the pituitary gland and in several limbic areas, was found to be greater in cyclic than in pregnant rats. However the enzyme activity did not change significantly during either the estrous cycle and pregnancy.