1. The characteristics of the electrical activity of beta-cells from islets of Langerhans recorded in vivo are described. For blood glucose concentrations from 4 to 11 mM, the electrical activity of pancreatic beta-cells is oscillatory, with alternating depolarized and hyperpolarized phases. During the depolarized phases, action potentials are triggered. 2. The main effect of increasing glucose concentration consists of an increase in the duration of the depolarized phase. The relationship between blood glucose concentration and the percentage of time in the depolarized phase can be described by a sigmoidal function with half-activation at 6.8 mM glucose. The equivalent value obtained in parallel experiments in vitro is 13.3 mM, a significant rightward shift in the activation curve that suggests a role for other neural or humoral factors in determining the islet sensitivity to glucose. 3. The injection of glucose into the bloodstream produces a transitory phase of continuous electrical activity that is recorded within seconds after the change and that leads to a decrease of the glycaemia to the prestimulatory value. 4. The results demonstrate that under physiological conditions the electrical response of beta-cells to glucose consists of membrane potential oscillations, validating previous data obtained with isolated preparations. Furthermore, the electrical response occurs at lower levels of glycaemia than those predicted from recordings in isolated preparations and is maximal within the physiological range of blood glucose.