The rat pancreas has frequently been used as an animal model to study changes in islet cells in pathological conditions, such as diabetes mellitus and islet cell tumours, but detailed quantitative data on the islets are not available. This study was therefore undertaken to investigate (1) the volume density of pancreatic islets, (2) islet diameter, islet volume and islet cell number and (3) islet cell pattern, i.e. the distribution, volume and number of each cell type per islet. The study also investigated the possibility of differences in various pancreatic regions derived from the dorsal primordium. The rat pancreas was divided into 4 regions: lower duodenal (derived from the ventral primordium) and upper duodenal, gastric and splenic regions (derived from the dorsal primordium). Sections were stained immunocytochemically with anti-insulin (B cells), antiglucagon (A cells), antisomatostatin (D cells) and antipancreatic polypeptide (PP cells) antibodies, and were used for morphometric analysis. A total of 1292 islets was examined, 328 from the lower duodenal, 245 from the upper duodenal, 314 from the gastric and 405 from the splenic regions. The mean volume density of the islets per pancreatic tissue was found to be 2.6 +/- 0.1%, 2.3 +/- 0.1%, 2.9 +/- 0.2% and 3.3 +/- 0.2%, in the lower duodenal, upper duodenal, gastric and splenic regions, respectively. The size-frequency distribution of the profile diameters of the islets showed an overall shift of all the size classes towards smaller sizes in the upper duodenal region, and towards larger sizes in the splenic region, as compared with the corresponding classes of the other regions.(ABSTRACT TRUNCATED AT 250 WORDS)