The effect of intravenous d- and dl-propranolol on serum insulin levels has been examined before and after the administration of glucose orally to 10 normal subjects. The increase in serum insulin 45 to 90 min after glucose ingestion was reduced during the infusion of either d- or dl-propranolol. The insulin suppression was not accompanied by any alteration in the glucose curves. The insulin/glucose ratios were reduced by both d- and dl-propranolol during the time of maximum glucose and insulin responses. Neither d- nor dl-propranolol induced any significant change in basal glucose or insulin levels before glucose administration. Since d-propranolol has no appreciable beta adrenoceptor blocking activity, it appears that the suppression of glucose-stimulated insulin release by propranolol may be due primarily to local anesthetic properties which are exerted equally by both isomers.