The present study investigated the effect of rat placental lactogen-II (rPL-II) on insulin secretion and B-cell proliferation of the maternal islets during the last half of pregnancy in rats using a homologous system. Pancreatic islets were isolated from nonpregnant rats and rats at day 13 of pregnancy and cultured for 8 days in medium containing trophoblast culture medium or purified hormones. The medium was changed daily and insulin concentrations were determined by measuring immunoreactivity. The number of proliferating B cells were determined by double staining for both insulin and 5'-bromo-2'-deoxyuridine (BrdU) incorporated into replicating DNA during the last 24 h of incubation. rPL-II-enriched medium, in which trophoblasts of placentae from rats at day 13 of pregnancy were incubated for 8 days, was added to the islet culture system. Insulin concentration in the medium of non-pregnant rat islets was significantly increased and doubled on incubation in 100% trophoblast culture medium. Addition of purified rPL-II to the culture medium of pregnant rat islets stimulated insulin secretion at the concentrations of 50-500 ng ml-1 in a dose-dependent manner. The stimulatory effect of rPL-II on insulin secretion was found to be more than double that with rat prolactin (rPRL) and rat growth hormone (rGH) at 100 ng ml-1. For determination of B-cell proliferation, non-pregnant rat islets were incubated with rPL-II, rPRL or rGH at 1000 ng ml-1 for 8 days and then 10 mmol BrdU l-1 was added during the last 24 h of incubation. rPL-II (5.30 +/- 0.36%), rPRL (3.79 +/- 0.34%) and rGH (2.87 +/- 0.29%) significantly increased the rate of incorporation of BrdU into the B cells compared with that of the control (1.34 +/- 0.18%). These results indicate that rPL-II directly regulates insulin secretion and B-cell proliferation of maternal islets during the last half of pregnancy in rats.