The effects of alpha- and beta-adrenoceptor blockade on plasma concentrations of insulin and glucose were studied in the anaesthetized rat. Infusion of the alpha 1-adrenoceptor blocker prazocin (80 micrograms/min), the alpha 2-adrenoceptor blocker yohimbine (15 micrograms/min) or the non-selective alpha-adrenoceptor blocker phentolamine (15 micrograms/min) during 50 min increased plasma insulin levels by about 1.5-2.5 ng/ml. The effects of phentolamine and prazosin on circulating insulin persisted throughout the infusion whereas the effect of yohimbine seemed to be more transient. Plasma glucose levels increased slightly during infusion of prazosin, but tended to decrease in response to phentolamine and yohimbine. The beta-adrenoceptor blocker propranolol (15 micrograms/min) lowered basal plasma insulin and glucose levels. It also depressed plasma insulin during infusion of all three alpha-adrenoceptor blockers without any appreciable influence on plasma glucose. It is suggested that both alpha 1- and alpha 2-adrenoceptor as well as beta-adrenoceptors are involved in the regulation of basal insulin secretion in the rat.