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Arginine is a potent stimulus to the secretion of placental lactogen (PL) as well as GH and prolactin in sheep. To determine whether other amino acids of the urea cycle also affect PL secretion, ornithine (25 g) and citrulline (20 g) were infused intravenously into four pregnant ewes over a period of 0·5 h. In eight experiments, ornithine stimulated PL secretion by 124 ± 25 (S.E.M.) % with the initial increase occurring by 1 h after the start of each infusion. Plasma GH concentrations increased from 3·4 ± 1·1 to 24·5 ± 5·9 ng/ml and plasma prolactin concentrations increased from 58·8 ± 12·8 to 310·7 ± 88·4 ng/ml. Citrulline, on the other hand, had no consistent effect on PL secretion. Plasma GH concentrations following the infusion of citrulline, however, increased by 15–20 ng/ml in two of four experiments and plasma prolactin concentrations decreased by 73·2 ± 3·2% in all four experiments.The results indicate that ornithine is also a potent stimulus to the secretion of PL, GH and prolactin and suggest that arginine-induced PL secretion may result from the conversion of arginine to ornithine.