Examination of livers from transgenic mice over expressing human growth hormone (hGH) revealed numerous alterations including a striking incidence of mitotic figures. The reason for increased proliferation is unclear, but could be related to effects of hGH, which is also acting as a lactogen in rodents. In order to identify some of the actions of GH, we have examined the effects of rat and human GH and rat prolactin on proliferation, as well as on morphological differentiation of normal rat liver cells in vitro. These cells, isolated from a 20-day-old rat, proliferate in culture, incorporate BrdU and are thus strikingly different from primary cultures of isolated hepatocytes, which typically are non-proliferating ceils. Monolayers of these cells were treated with rat prolactin (rPRL), rat growth hormone (rGH). rPRL and rGH in combination, or hGH, for 24 hr. Subsequently, mitotic figures were counted and the cultures were processed for transmission electron microscopy. The incidence of mitotic figures was significantly increased by rPRL (27.4%) versus control (19%), while rGH (13%) and hGH (9.6%) significantly decreased proliferation. In controls, 2% of the proliferating cells were in prophase, approximately 12% in metaphase and approximately 15% in telophase. In contrast, rPRL caused a significant increase in the number of cells in prophase (14%) and reduced the number of cells in the other mitotic stages. hGH and rGH reduced the overall number of mitotic figures. Unexpectedly, the effects of rGH plus rPRL were different from the effect of hGH. In addition, each treatment caused distinct morphological changes of liver cell organelles. Thus, after rPRL treatment mitochondria were generally large and bent, while rGH treatment was associated with irregularly shaped mitochondria and conspicuous microtubule/ microfilament arrangements. In contrast, after rPRL plus rGH treatment the chromatin pattern was altered. In summary, these results suggest specific hormone effects on both cell proliferation and metabolism as reflected by distinct changes in morphology. Unexpectedly, the combined effect of rGH and rPRL were not identical to the effects of hGH, suggesting a possibility of a unique effect of hGH in this system.