We report a highly sensitive electrochemical approach suitable for the real time measurement of insulin release from single islets of Langerhans, the functional endocrine units in the pancreas. The method is based on the detection of the insulin surrogate 5-hydroxytryptamine (5-HT) by carbon fibre microelectrodes implanted in the islets. Based on the combination of this novel approach with the simultaneous microfluorometric recording of cytosolic free Ca2+ concentration ([Ca2+]i), we demonstrate that glucose-stimulated islets secrete 5-HT/insulin in a pulsatile fashion under physiological conditions, and that this activity is encoded by synchronous [Ca2+]i oscillations. The sensitivity to detect variations in minute amounts of secreted materials is partially conferred by the fact that the tracer is released into a relatively confined space (the intraislet interstitial space).