For cows on d 137 (n = 6), 180 (n = 8), 226 (n = 9) and 250 (n = 5) of gestation (Exp. 1), concentrations of insulin and glucose were two- to three-fold less (P less than .01) in fetal venous plasma than in uterine arterial plasma. Concentrations of growth hormone, conversely, were 10- to 20-fold greater (P less than .01) in fetal venous than in uterine arterial plasma. Concentrations of insulin and glucose in maternal and fetal plasmas and concentrations of growth hormone in maternal plasma did not vary with stage of gestation. Concentrations of growth hormone in fetal venous plasma, however, were greater on d 226 and 250 than on d 137 and 180. For cows (n = 6) on d 198 of gestation (Exp. 2), concentrations of insulin and glucose in maternal and fetal plasmas and of growth hormone in maternal plasma remained relatively constant in samples collected every 30 min for 3 h. In contrast, growth hormone concentrations in fetal venous plasma were highly variable and appeared to be episodic, with pulses of 10 to 60 ng/ml in amplitude. No significant correlations were found among concentrations of insulin, glucose and growth hormone in fetal venous plasma. When samples were collected every 15 min for 4 h from cows (n = 5) on d 198 of gestation (Exp. 3), episodes of growth hormone in fetal venous plasma were irregular in amplitude and frequency.(ABSTRACT TRUNCATED AT 250 WORDS)