A mathematical model that represents the dynamics of intracellular insulin granules in -cells is proposed. Granule translocation and exocytosis are controlled by signals assumed to be essentially related to ATP/ADP ratio and cytosolic Ca²⁺ concentration. The model provides an interpretation of the roles of the triggering and amplifying pathways of glucose-stimulated insulin secretion. Values of most of the model parameters were inferred from available experimental data. The numerical simulations represent a variety of experimental conditions, such as the stimulation by high K⁺ and by different time courses of extracellular glucose, and the predicted responses agree with published experimental data. Model capacity to represent data measured in a hyperglycemic clamp was also tested. Model parameter changes that may reflect alterations of -cell function present in type 2 diabetes are investigated, and the action of pharmacological agents that bind to sulfonylurea receptors is simulated.