Hereford steers (290 +/- 6 kg of BW) were implanted (n = 4) with 140 mg of trenbolone acetate (TBA) and 28 mg of estradiol-17 beta (E2 beta) or nonimplanted (controls, n = 4). In Trial 1, effects of a single i.v. injection of 0, 20, 40, or 80 micrograms of a growth hormone-releasing factor (1-29 NH2) analogue (GRFa) on release of endogenous somatotropin (ST) were evaluated in a double 4 x 4 Latin square design. Plasma samples (n = 21) were obtained from -20 to 240 min after GRFa injection. Area under the ST response curve (AUC) increased (P = .009) in a dose-dependent manner (.2, 2.6, 3.6, 4.3 mg.min-1.mL-1, respectively). Mean ST concentration was not affected (P = .238) by implant but AUC was greater (P = .009) in implanted than in control steers. There was no interaction (P = .460) between dose of GRFa and presence of implant. In Trial 2, 80 micrograms of GRFa was administered at 12-h intervals to the same eight steers. Response of ST (AUC) to the first and last (13th) i.v. injection of GRFa was similar and not affected by implant. Before GRFa administration, plasma insulin-like growth factor I (IGF-I) concentrations were greater (P = .039) in implanted than in control steers (272 vs 164 ng/mL). Administration of GRFa increased plasma IGF-I (P = .0001), decreased plasma urea N (PUN) (P = .0001), and did not alter plasma glucose (P = .447) in both control and implanted steers. Data indicate that effects of GRFa and TBA/E2 beta on plasma IGF-I and PUN concentrations were additive in this study.