Lactogenic hormones, PRL and placental lactogen, are important regulators of insulin secretion and islet beta-cell proliferation. In this study we examined the presence of PRL receptor immunoreactivity in pancreatic islets of Langerhans using PRL receptor monoclonal antibodies provided by Dr. Paul Kelly. Studies were performed using islets isolated from neonatal, adult, and day 14 pregnant rats. The islets were examined by immunohistochemistry and laser scanning confocal microscopy. In neonatal rat islets, PRL receptors were observed in beta- and alpha-cells, but not in delta-cells. Among islet beta- and alpha-cells there was heterogeneity of cellular staining for PRL receptors. A small portion of the cells was intensely stained for PRL receptors. However, the majority of the cells had a much lower level of staining intensity, suggesting that most islet cells have a low level of PRL receptors. In general, alpha-cells were more uniformly stained than beta-cells. Similar results were obtained with adult rat islets, in which, again, there was a large range of staining intensity and many cells with low levels of PRL receptor. Rats on day 14 of pregnancy had an increased level of islet PRL receptor expression compared with age-matched control animals. There was also a decrease in cellular heterogeneity for PRL receptors, with nearly all cells having a uniformly high level of PRL receptor expression. JAK2, the tyrosine kinase associated with PRL receptors, was examined in Nb2 cells and islets. JAK2 immunoreactivity was detected at the cell membrane in very low levels in Nb2 cells. It was also found in numerous vesicular structures in the cytoplasm, where it colocalized with PRL receptors. A prominent feature of all cells was the presence of JAK2 in the nucleus, but not the nucleolus. In islets, JAK2 immunoreactivity was similarly observed in the nucleus of nearly all cells. However, the vesicular cytoplasmic location of JAK2 was less frequently observed and did not colocalize with PRL receptors. For comparison, JAK2 immunoreactivity was examined in several other tissues where it was detected in fibroblasts (endomysial and endoneurial cells), smooth muscle cells, and ganglion cells in the pancreas. JAK2 was notably absent from pancreas acinar cells, hepatocytes, skeletal muscle cells, and Schwann cells. This study demonstrates the presence of PRL receptors in islet beta- and alpha-cells, but not delta-cells. There was an increase in PRL receptor expression in islets during pregnancy, which is commensurate with the up-regulation of islet function. In addition, JAK2 immunoreactivity was detected in most islet cells and Nb2 node cells.