The effects of chronic (3 mg/day for 1 week) administration of the vasodilator drug prazosin on several metabolic and endocrine variables were evaluated in 12 hypertensive patients, 6 with normal and 6 with abnormal oral glucose tolerance test (OGTT). After 1 week prazosin treatment there were no significant modifications in fasting plasma glucose, serum free fatty acids (FFA), cholesterol, triglycerides, insulin (IRI), growth hormone (GH), prolactin (PRL) and gastrin levels; oral glucose tolerance and IRI response to glucose were unchanged in normal subjects, while in chemical diabetics there was a significant improvement in glucose tolerance and a slight increse in IRI secretion. Therefore, the untoward metabolic effects of acute prazosin administration, i.e. increased plasma glucose and serum FFA, are not sustained during chronic treatment, which may even improve glucose metabolism in diabetic patients.