Studies with phentolamine, an alpha-adrenergic antagonist, in normal subjects and diabetic patients have indicated that insulin secretion may be inhibited by tonic alpha-adrenergic stimulation of pancreatic B-cells. We evaluated, with the use of the highly selective alpha 2-adrenoceptor antagonist idazoxan, the role of alpha 2-adrenergic receptors in the regulation of glucose-induced insulin secretion. A glucose infusion test (GIT) was performed after the administration of idazoxan or placebo in normal men (n = 15) and men with noninsulin-dependent diabetes mellitus (n = 6). The normal men were divided into two groups on the basis of high (n = 8) and low (n = 7) insulin responses to prior GITs. The blood glucose and plasma insulin and C-peptide responses to the GIT were similar after idazoxan (40 mg, orally) or placebo treatment in all three groups, although the responses differed among the groups. In the diabetic group iv administration of idazoxan 20 min before the GIT did not alter the insulin response to the GIT. We conclude that alpha 2-adrenergic blockade does not affect glucose-induced insulin secretion in normal men, nor does it improve the impaired first phase of insulin secretion in low insulin responders and noninsulin-dependent diabetes mellitus patients. Phentolamine probably stimulates insulin secretion by a mechanism not involving alpha 2-adrenergic receptors directly.