The major changes in pancreatic islet function during pregnancy and after exposure to lactogens are an increase in β-cell proliferation and enhanced insulin secretion. In this study we examined INS- 1 cells as a potential model for further inquiry into PRL signaling in β-cells. Proliferation of β-cells, insulin secretion, and quantitative immunocytochemical analysis of STAT5 translocation were studied. PRL treatment of INS-1 cells resulted in a 2- to 4-fold increase i cell proliferation compared to that in the control group. In contrast, there was no effect of PRL treatment on HIT cell proliferation and only a very small effect on RIN cell proliferation. A significant effect on INS-1 cell proliferation was observed at 10 ng/ml and reached a maximum at 200 ng/ml. PRL treatment resulted in enhanced insulin secretion from INS-1 cells. There was a time-dependent increase in insulin secretion, which when corrected for cell number was 1.5-fold greater in the PRL-treated cells. The effects of PRL on cell division and insulin secretion were glucose dependent. The presence of the JAR family of tyrosine kinases and the transcription factor STAT5 in INS-1 cells was examined by immunocytochemical techniques. Although all members of the JAK family of kinases were detected, the staining intensity of JAK-2 was noticeably more intense. Initial studies of STAT5 translocation were performed using PRL-dependent Nb2 lymphoma cells, in which PRL treatment resulted in a nearly complete translocation of cytoplasmic STATS to the nucleus. Under control conditions there was a near-equal fluorescence intensity of STAT5 staining in the nucleus and cytoplasm of INS-1 cells. PRL treatment resulted in a time-dependent increase in STAT5 staining in the nucleus, with a corresponding decrease in the cytoplasm. The STAT5 staining intensity in the nucleus remained elevated for the duration of PRL treatment. This effect was reversible upon removal of PRL from the medium. Besides PRL, both GH and FBS induced a similar translocation of STAT5 to the nucleus. Although present in RIN cells, no detectable changes in STATS were observed in RIN cells after exposure to PRL, GH, or FBS. INS- 1 cells should provide a good model for further inquiry into the intracellular signaling pathways used by PRL and how these events alter islet function.