The aim of the present study was to investigate possible changes in the islet microvasculature during the period of pronounced beta-cell growth seen perinatally in rats. We studied islet endothelial and beta-cell proliferation, as well as islet vascular density, in rats during this period. There was a progressive increase in islet vascular density from day -1 to day 7 postpartum, with values similar to those in adult rats seen at the latter time point. (3)H-thymidine-labelled islet endothelial cells were extremely rare in adult rats, whereas such cells were much more frequent perinatally. The beta-cell labelling index was higher in all perinatal animals than in adult rats, with peak values seen on day 2. The proliferating endocrine cells were located very close to blood vessels at day 2 after birth. In conclusion, the pronounced growth of islet endocrine cells seen during the first week after birth coincides and co-localizes with an even more pronounced increase in islet endothelial cell proliferation, which results in a marked increase in intra-islet vascular density. This perinatal increase in islet blood vessel density may facilitate glucose sensing and islet hormonal delivery to the systemic circulation.