Although the effects of inhibin on gonadotropin synthesis and secretion have been extensively studied, the role of inhibin in the neuroregulation of in vivo growth hormone (GH) secretion still remains to be elucidated. In the present work, we investigated the effects of inhibin on spontaneous GH secretion in three different groups of conscious adult male rats: intact, gonadectomized, and dihydrotestosterone (DHT)-treated gonadectomized animals. We found that inhibin administration (100 µg/kg, i.v.) led to a marked suppression in spontaneous GH secretion in all the groups studied. This significant decrease was assessed by the area under the curve in intact (311.1 ± 163.3 vs. 3,882.1 ± 1,084.6 ng/ml/6 h, p < 0.01), gonadectomized (416.6 ± 120.9 vs. 2,078.5 ± 298.4 ng/ml/6 h, p < 0.01) and gondadectomized rats treated with DHT (755.0 ± 102.3 vs. 4,539.3 ± 1,670.6 ng/ml/6 h, p < 0.01). Furthermore, intravenous inhibin significantly reduced in vivo GH responses to GHRH (10 µg/kg, i.v.) in both intact and gonadectomized rats. These findings suggest a role of inhibin on in vivo GH secretion in the male rat.