Effects of bovine and human growth hormone-releasing hormone (GRF) analogs (bGRF(1-29)-NH₂: bGRF-29, [D-Ala², Ala¹⁵]-bGRF-29, [D-Ala²]-hGRF(1-29)-NH₂: [D-Ala²]-hGRF-29), bovine GRF (bGRF(1-44)-NH₂: bGRF-44), as well as rat GRF (rGRF) on GH release were studied in female calves. Intravenous (iv) bolus injections of 0.25μg/kg BW of bGRF-29, [D-Ala², Ala¹⁵]-bGRF-29, and [D-Ala²]-hGRF-29 stimulated GH release. Plasma GH levels began to rise 10min after the injection of each peptide, and significant increases in GH concentrations were obtained at 60, 180 and 150min after the injection of bGRF-29, [D-Ala², Ala¹⁵]-bGRF-29 and [D-Ala²]-hGRF-29, respectively. The concentrations of GH 80min after the injection of [D-Ala², Ala¹⁵]-bGRF-29 were significantly higher than those after the injection of [D-Ala²]-hGRF-29 (except at 80 and 90min) or bGRF-29. The iv bolus injections of 0.25μg/ kg BW of bGRF-44 and rGRF stimulated GH release, and the GH-releasing potency of rGRF was approximately equal to that of bGRF-44. The plasma GH responses to the repeated iv injection of bGRF-29 or [D-Ala², Ala¹⁵]-bGRF-29 at 2-h intervals were examined. bGRF-29 acutely increased plasma GH levels after each injection, and the high GH levels decreased to the basal values within 2h. In contrast, high GH levels induced by [D-Ala², Ala¹⁵]-bGRF-29 were gradually decreased but not lowered to basal values throughout the experiment. These results show that [D-Ala², Ala¹⁵]-bGRF-29 has longer-lasting and greater GH-releasing activity than the other GRF analogs in female calves, and the GH-releasing potency of rat GRF is approximately equal to that of bovine GRF in cattle in vivo