The inhibitory effect of dietary supplementation of certain retinoids on mammary carcinogenesis in the rat has been reported from our laboratory. Specific cytosolic retinoic acid binding proteins (cRABP) as well as retinol binding proteins sedimenting as 2S components have been detected in the mammary tissue during normal and neoplastic differentiation. Relatively higher levels of cRABP were observed in the mammary glands from pregnant animals as well as in ovarian hormone independent tumors; whereas in glands obtained from lactating rats and in hormone dependent tumors, lower levels of cRABP were evident. Exogenous treatment of such animals with estradiol-17 beta enhanced the levels of cRABP. The results indicate a possible correlation between endocrine and retinoid function in both normal and neoplastic differentiation of mammary tissue. (3H) retinoic acid-RABP complex, under appropriate conditions, translocates into the nucleus. Unbound (3H) retinoic acid, however, failed to associate with the nuclear sites.