The direct effects of human GH (hGH), ovine pituitary PRL (oPRL), and human chorionic somatomammotropin [placental lactogen (hPL)] on the endocrine pancreas were studied in isolated pancreatic islets maintained in tissue culture. Islets of Langerhans were isolated by collagenase treatment of pancreatic tissue obtained from adult NMRI mice and adult or newborn Wistar rats. The islets were maintained for up to 3 weeks in petri dishes containing tissue culture medium RPMI 1640 supplemented with newborn calf serum or normal human serum. The release of insulin during culture and the islet content of insulin, glucagon, and DNA after culture were determined. The DNA synthesis in the newborn rat islets was evaluated by the incorporation of [methyl-3H]thymidine into islet cell DNA. In mouse islets, 1 micrograms/ml hGH, oPRL, or hPL markedly stimulated insulin release during a 2-week culture period and caused a significant increase in the insulin content in the islets after culture. While hGH did not affect the DNA content in adult mouse islets, an increase was observed in adult rat islets after 2-3 weeks of culture. In islets isolated from 3- to 5-day-old rats cultured for 2 weeks with hGH, there was a 30-40% higher DNA content than that found without hGH. Correspondingly, a significant stimulation of the incorporation of [methyl-3H]thymidine could be demonstrated 24 h after the addition of hGH, oPRL, or hPL. hCG and porcine ACTH had no effect. In conclusion, these results indicate that GH and related hormones have a direct stimulatory effect on both the insulin production and DNA synthesis in isolated islets of Langerhans. Whether the effect is directly on the beta-cell or mediated via locally produced growth factors remains to be determined.