The effects of tolbutamide on insulin release, 45Ca2+ uptake and 86Rb+ efflux were studied in isolated rat islets. At a low glucose concentration (75 mg/dl), tolbutamide (20-500 microgram/ml) produced a rapid, dose-dependent increase in insulin release from perifused islets. After 30-40 min however, the rate of secretion as well as the potentiating effect of theophylline were inversely related to the concentration of sulphonylurea. The monophasic release of insulin triggered by tolbutamide (100 microgram/ml) at low glucose could be evoked again by removing and reintroducing the drug, or by temporarily withdrawing calcium or adding cobalt to the medium. Tolbutamide (20 microgram/ml) accelerated and potentiated the biphasic insulin release in response to a secondary stimulation by glucose (150 mg/dl). By contrast, 100 microgram/ml tolbutamide reduced the releasing effect of glucose to a slow increase in secretion rates. Theophylline normalized the second phase of release, but did not restore the rapid phase. Tolbutamide stimulated 45Ca2+ influx (2 min-uptake) in islet cells; this effect was maximum immediately after addition of the drug and decreased later on, exhibiting a monophasic pattern. Glucose stimulation of Ca2+ uptake (5 min) was reduced in the presence of 100 microgram/ml tolbutamide. At a low glucose concentration, tolbutamide reversibly reduced 86Rb+ efflux (tracer of K+) from islet cells, without altering the further inhibition of this efflux by a later glucose increase. It is suggested that tolbutamide depolarizes B cells partially by reducing their K+ permeability. This depolarization leads to opening of voltage-dependent calcium channels and the resulting Ca2+ influx triggers insulin release. The important and maintained depolarization by high concentrations of tolbutamide may secondarily inactivate these channels and cause a decrease in Ca2+ influx. This could explain the monophasic release of insulin and the refractoriness of B cells to subsequent glucose stimulation.