Blood plasma PRL, GH and glucocorticoids in heifers increased severalfold within 5-15 min after im injections of 15, 30, or 60 mg PGF2α or after a single 5 mg iv injection of PGF2α. Plasma LH increased at least twofold over basal estimates but not until 1.5-6 hr after im PGF2α. Constant iv infusion of PGF2α at the rate of 0.5 mg/min for 30 min produced greater plasma concentrations of PRL, GH, glucocorticoids and LH than those found after im injections. Plasma glucose increased 59-67% above pretreatment values between 30 and 60 min after iv administration of PGF2α. Plasma insulin increased more than twofold over basal estimates at 45 min after PGF2α administration, but these increases only approached significance (P ≃ 0.10). Free fatty acids did not change significantly (P > 0.05) after PGF2α administration. While exogenous PGF2α administration causes marked increases in several plasma hormones and glucose, the mechanism of action is unclear.