Immunoreactive growth hormone (GH) secretion was measured in an in vitro system containing rat hemipituitaries. Sodium pentobarbital 4.0mM (1 mg/ml) and 16.0mM (4 mg/ml) in the incubation medium produced a stimulation of GH release of 30% and 100% when compared to contralateral hemipituitaries in the medium alone, and a net decrease in pituitary GH content. Pairs of hemipituitaries incubated in either sodium pentobarbital 16.0 HIM or equimolar sodium chloride were compared. Baseline secretion in buffer alone was 148 ng GH/mg pituitary/hr, in sodium pentobarbital + buffer, 360 ng/mg/hr, and sodium chloride + buffer, 162 ng/mg/hr. All pituitary halves exposed to 3.3 HIM theophylline for a further hour demonstrated the capacity to respond with increased secretion compared to baseline (mean 384 ng/mg/hr). These results demonstrate an unexpected direct stimulatory effect of sodium pentobarbital on the secretion of GH from the pituitary in the rat. (Endocrinology91: 1513, 1972)