The B cells of the islets of Langerhans in the rat have been studied with regard to whether they were situated centrally or peripherally in large islets or in small islets. Regional differences in the nuclear size and the position of the nucleoli within the nuclei were found. The smallest nuclear projection areas and the lowest number of nucleoli adjacent to the nuclear membrane were recorded centrally in large islets. The cytological differences may be interpreted as a sign of corresponding differences in the functional state of the B cells. A lower activity of the central B cells in a large islet may be due to these cells being older or the concentrations of glucose and insulin being different from those at the periphery or in small islets.