After a step-function increase in glucose concentration, insulin secretion by perifused isolated rat islets of Langerhans showed oscillations superimposed on the well-known first- and second-phase secretory components. The oscillations were sustained for the length of the experiment and corresponded to at least four cycles. This established the existence of an oscillatory pacemaker with a narrow dispersion of periodicities intrinsic to the islets and showed that synchronization of islet action could be achieved by a step-function increase in glucose concentration. The observed period of 16 min is similar to the period of oscillatory insulin secretion in a number of intact organisms. This argues for identity of pacemakers in vivo and in isolated islets. This means that neural or other forms of interislet communication are not prerequisites for oscillatory insulin secretion. Theophylline increased the length of the oscillatory period, suggesting the periodicity of the pacemaker of insulin secretion can be metabolically regulated. This observation also provided a basis for explaining fine tuning of oscillatory periods by the nervous system.