The atherogenic effect of pharmacologic amounts of estrogen on young cockerels has been quantitated through time and dose response studies. The assay consists of intramuscular administration of 2.5 mg of estradiol cyclopentylpropionate in cottonseed oil (ECP) at beginning of a 7-day period. Feminization and artery changes appear to result from ECP dosage levels lower than required to produce blood serum changes associated with atherosclerosis.