The effects of diltiazem hydrochloride on insulin secretion and glucose tolerance were studied in diabetic subjects treated with dietary management alone for their glycemic control. Sixteen diabetic subjects with heart complaints and/or abnormal electrocardiographic findings which were compatible with diagnosis of ischemic heart disease were given diltiazem hydrochloride 90 mg daily for a mean of 9.3 +/- 1.0 (SE) months and oral 100 g glucose tolerance tests (o-GTT) were performed before and after the administration of diltiazem hydrochloride. As a control group, 14 diabetic patients with roughly the same level of fasting blood glucose and the same degree of glucose intolerance were selected, and in these patients oral 100 g glucose tolerance tests were also performed before and after a mean follow up period of 12.6 +/- 1.5 months. Although improvement in glucose tolerance was observed at the end of the trial in the group of patients with diltiazem hydrochloride, control group also showed improved glucose tolerance and there was no significant difference in the change of glucose tolerance between both groups. No significant change of IRI response in oral 100 g glucose tolerance test was observed in both groups. We could, therefore, conclude from these observations that diltiazem hydrochloride with usual clinical dosage dose not exert unfavourable effects on glucose tolerance and insulin secretion in the mild diabetic patients.