Intravenous glucose and/or tolbutamide administered in two consecutive pulses 30 and 60 min apart to the same subjects using identical doses showed that insulin secretory responses was altered during a subsequent stimulation and that this was modulated by the time factor. Insulin response was more sustained after the second glucose pulses and the insulin peak response was delayed and diminished if the second glucose dose was given 30 min after the first, but not if given 60 min later. It is suggested that the beta-cell membrane might remain partially depolarized above a certain glucose level or that a postulated signal relay mechanism might become saturated. Responses to two tolbutamide pulses did not show these characteristics; however, the second insulin response was smaller than the first. When the first pulse was glucose and the second tolbutamide, or vice versa, the second responses were altogether different from those elicited by the double doses of either tolbutamide or glucose. To explain these characteristic patterns of insulin secretory dynamics, the existence of occult glucose receptors on the beta-cell that are opened up by tolbutamide was postulated. These studies do not support the two-pool theory, or at least restrict it to glucose-stimulated insulin response. The positive correlations between the first and the second insulin responses in all tests argue strongly against the existence of an insulin feedback mechanism in man.