Blood glucose, plasma GH, insulin and glucagon levels during oral glucose tolerance test before and after treatment with bromocriptine (5-20 mg daily for 2-9 months) were investigated in eleven acromegalic patients with glucose intolerance. Nine out of 11 patients showed improvement or normalization in glucose tolerance after bromocriptine therapy. Basal levels of plasma GH were markedly decreased in 7 of 11 patients treated, although the improved glucose tolerance was not always associated with a drop of basal plasma GH levels. In contrast, basal plasma glucagon level showed a distinct fall in all the patients whose glucose tolerance was improved, but unchanged in whom glucose tolerance was not ameliorated. The mean values of plasma GH and glucagon after oral glucose load were significantly lower during bromocriptine therapy than those before the treatment, respectively. Basal levels of plasma insulin and its response to glucose load did not change after bromocriptine treatment. Bromocriptine thus appears to be a good alternative in the treatment of glucose intolerance in acromegalic patients and the improvement of glucose tolerance by bromocriptine may be related to the reduction in plasma glucagon levels. The possibility, however, is not excluded that a decrease by bromocriptine in the total daily GH secretion is a cause of the improved glucose tolerance.