Female rats were administered increasing doses of morphine sulfate 5 days prior to mating and during gestation until 4-6 days before the birth of their litters. Prenatal morphine exposure altered the normal during the 3rd and 4th postnatal weeks. This disruption in behavioral ontogeny did not coincide with changes in physical parameters. Decreased body weight ant postnatal week. The appearance of behavioral disturbances in the absence of physical abnormalities stresses the need for follow-up studies of infants born to narcotic-dependent mothers after signs of physical withdrawal or retarded growth have disappeared.