Since progesterone is known to modulate the effects of estradiol in a number of organ systems, we have studied the effects of the two hormones on very low density lipoproteins (VLDL) synthesis in the cockerel. We have also examined the effect of an estrogen antagonist, nafoxidine-HCl, on the process. When three week-old cockerels were treated with a single injection of estradiol (1 mg) or estradiol (1 mg) + progesterone (2 mg), plasma VLDL increased within 5 h, reached a peak at 48 h and returned to baseline values at 68 h. There was no significant difference between estradiol or estradiol + progesterone treated animals. Liver slices were prepared from similarly treated animals and incubated in vitro with L-[3H]lysine for 2 h. Following homogenization and centrifugation at 105,000 X g, newly synthesized VLDL were quantitiated in the supernatant fluid by immunoprecipitation with a monospecific antibody. Radioactivity incorporated into VLDL was found to increase from low baseline levels to a peak at about 17 h after treatment. There was, again, no significant difference between estradiol and estradiol + progesterone-treated animals. Estrogen priming or progesterone pretreatment also did not significantly alter the VLDL biosynthetic response in liver slices to estradiol alone or in combination with progesterone. When the estrogen-antagonist nafoxidine-HCl (5 mg) was administered simultaneously with estradiol (1 mg), it totally inhibited the VLDL biosynthetic response in liver slices in vitro. The interaction of estradio, progesterone and nafoxidine-HCl on the hepatic synthesis of VLDL should serve as a valuable model for the study of VLDL synthesis and its regulation as well as the mode of action of the sex steroid hormones in the liver.