It is unclear whether reported histopathological changes in the endocrine pancreas of the GK rat (a spontaneous model of non-insulin-dependent diabetes) are related to the pathogenesis of hyperglycaemia or occur secondarily to metabolic alterations. We found that total pancreatic insulin stores in GK rats from the Paris colony were depleted by 62% (p < 0.01) in adult (4-month-old) overtly hyperglycaemic animals compared to those of normal Wistar control rats, and that beta-cell mass in GK pancreata was decreased to a similar extent (51%, p < 0.05). This indicates that decreased in vivo and in vitro insulin secretory response to glucose in GK rats could be due not only to impaired stimulus-secretion coupling for glucose in their beta cells but also to a reduced number of beta cells. Reduced total beta-cell mass in adult GK rats was associated with a noticeable alteration in the architecture of a subpopulation of islets: only large islets displayed signs of disorganization of the mantle-core relationship due to prominent fibrosis, with clusters of beta cells widely separated by strands of connective tissue. Our study also provides a first record of the pathophysiologic changes occurring in the GK rat from the neonatal period. Four-day-old GK pups demonstrated normal basal glycaemia compared to Wistar rats of the same age. GK islets displayed a well-preserved architecture, with normal staining of beta cells and no fibrosis. However, their total pancreatic insulin stores and total beta-cell mass were significantly lower [59% (p < 0.01) and 64% (p < 0.05) respectively] than those of controls. These data indicate that a reduction in islet tissue clearly predates the onset of diabetes (hyperglycaemia). Therefore, a reduction of total beta-cell mass should be considered as a primary feature in the pathological sequence leading to diabetes in GK rats, at least in those originating from the Paris colony.