Vitellogenesis was induced in mature male Japanese quail following intramuscular injections of 16 mumol/100 g body weight of any one of four estrogen analogues or 160 mumol/100 g of the nonsteroid zearalenone. Six hours after the injections, microgram levels of vitellogenin were detected and quantitated by rocket immunoelectrophoresis. Sodium dodecyl sulfate polyacrylamide gel electrophoresis of plasma from estrogen-treated birds and from 160 mumol zearalenone-treated birds showed, 4 days after the injections, the three subunits of vitellogenin normally seen in the plasma of egg-laying female quail. As evidenced by increased concentrations of plasma protein-bound phosphorus, total plasma calcium, and total plasma protein, mole-for-mole diethylstilbestrol dipropionate was 114%, ethynyl estradiol 75%, moxestrol 108%, and zearalenone 5.6% as effective as estradiol-17 beta in inducing vitellogenesis over a 4-day period. The responses to 160 mumol zearalenone/100 g over 8 days were approximately .6 the responses for estradiol. The responses to ethynyl estradiol were about equal to those for estradiol for days 1 and 2, then faded to .3 the estradiol response by day 8. Doses of 16 or 160 mumol/100 g of cholecalciferol, chlordecone, corticosterone, o,p1-DDT [1,1,1-trichlor-2-(p-chlorphenyl)-2-(o-chlorphenyl)eth ane], methoxychlor, progesterone, or testosterone, or 16 mumol/100 g of zearalenone did not induce vitellogenesis. Induction of vitellogenesis in male Japanese quail can thus provide a sensitive test of the estrogen-mimicking activity of some possible environmental contaminants.