Potencies of human pancreatic growth hormone-releasing factor [hpGRF(1–40)-OH] and of a peptide corresponding to the N-terminal 29 residues of rat hypothalamic GRF, [rGRF(1–29)-NH 2] were compared in two experiments. Eight Angus steers averaging 297 days of age and 290 kg in February 1984 were used in Exp. 1. Five months later six of the steers, weighing 391 kg, were used in Exp. 2. In Exp. 1, hpGRF(1–40)-OH and rGRF(1–29)-NH 2 were infused for 5 min at rates of 0, 1.3, 2.6, 5.2, 7.8 and 13.3 pmol/min/kg. Two steers were infused simultaneously, one received hpGRF(1–40)-OH and the other the equivalent dose of rGRF(1–29)-NH 2. Four pairs of steers received each dose. Both peptides elicited rapid GH release. Plasma GH concentrations peaked 15 to 20 min following onset of GRF administration, and returned to baseline levels 60 to 90 min later. Minimum effective doses, the lowest dose tested that resulted in a statistically significant GH reponse, were 5.2 pmol/min/kg hpGRF(1–40)-OH and 13.3 pmol/min/kg rGRF(1–29)-NH 2. Magnitudes of GH responses to 5.2, 7.8 and 13.3 pmol/min/kg hpGRF(1–40)-OH and 13.3 pmol/min/kg rGRF(1–29)-NH 2 were similar; corresponding to respective peak concentrations of 79, 66, 57 and 56 ng/ml. Growth hormone levels before GRF administration averaged 16 ng/ml. Experiment two was designed like the first except steers were infused for 6 hr with hpGRF(1–40)-OH and rGRF(1–29)-NH 2 at rates of 0, .5 and 1 pmol/min/kg. Both peptides at both rates raised (P<.05) GH concentrations during the 6 hr infusion period. Mean GH levels were 7 ng/ml during saline infusion, 30 and 23 ng/ml during infusion of .5 pmol/min/kg hpGRF(1–40)-OH and rGRF(1–29)-NH 2, and 41 and 27 ng/ml during infusion of 1 pmol/min/kg of the respective peptides. The initial GH response was biphasic, after which GH levels decreased temporarily and then one or two more GH surges occurred during the latter portion of the infusion period. Results demonstrate that hpGRF(1–40)-OH and rGRF(1–29)-NH 2 are potent GH secretagogues in steers. Potency of rGRF(1–29)-NH 2 is about 40% of hpGRF(1–40)-OH. Intrinsic activities, their ability to stimulate maximum GH secretion, appear to be similar. Both peptides are effective in raising GH levels over a 6 hr constant infusion period.