MK-0677, a spiroindoline sulfonamide, is a novel, orally active GH secretagogue. The effects of MK-0677 on serum GH and other hormones after oral and iv single dose administrations in beagles were evaluated. After oral administration in a balanced eight-dog crossover study, treatment with MK-0677 significantly increased peak GH concentrations, with a 5.3-fold increase (mean +/- SEM, 10.5 +/- 1.9 ng/ml) at the 0.25 mg/kg dose, a 9.0-fold increase (18.0 +/- 3.3 ng/ml) at the 0.50 mg/kg dose, and a 15.8-fold increase (31.6 +/- 5.8 ng/ml) at the 1.0 mg/kg dose. Total GH release, expressed as the area under the curve, showed similar significant increases over the effect of the water placebo. A single oral 1 mg/kg dose in three dogs induced a mean GH peak of 27.6 +/- 1.5 ng/ml at 120 min, and GH levels remained elevated up to 360 min after treatment. Insulin-like growth factor I (IGF-I) levels were significantly increased by 30% at 480 min after treatment. Cortisol levels were increased 2.4-fold over pretreatment levels. After i.v. administration, compared to the saline control group which had a mean (+/- SEM) serum GH peak of 3.8 +/- 0.7 ng/ml, MK-0677 at 0.25 mg/kg significantly increased (P < 0.05) peak GH concentrations 20.4-fold (77.4 +/- 13.7 ng/ml). Total GH release, expressed as the area under the curve, showed a similar increase. The mean peak GH level was recorded 10 min after treatment, with GH levels elevated up to 180 min after treatment. IGF-I levels were significantly elevated by 25% 360 min after the administration of MK-0677. Cortisol levels were increased 2.3-fold over pretreatment levels. Insulin and glucose levels were higher, LH and PRL levels were unaltered, and T4 levels were marginally lower; the levels of each of these hormones remained within the normal ranges for dogs throughout the experiment. In summary, MK-0677 is a potent GH secretagogue that induces an immediate, large, long lasting increase in GH levels when administered orally or i.v. In contrast to GH-releasing peptide-6 and benzolactam secretagogues, GH levels were elevated up to 360 min after treatment, and this was associated with a significant increase in IGF-I levels. Cortisol levels were increased; however, the increases were modest compared to those in GH.