The aim of this study was to evaluate the acute hepatic effects exerted by the steroid hormone progesterone (PR) in the rat. Although the liver is not a target tissue for this hormone, a number of hepatic actions of PR have been described, and, furthermore, a specific binding site for PR (PBS) exists in rat liver microsomes. Immature male rats were treated intraperitoneally with 60 mg/kg PR per day for 1, 5 or 10 days, and different parameters were evaluated in order to detect possible alterations in liver cells. Morphological study of the livers did not present images of cytotoxicity in any group of animals. The presence of a clear hyperplasia of smooth endoplasmic reticulum (SER) was noteworthy, mainly seen in perilobular hepatocytes. Despite this SER increase, the levels of cytochrome P450 (Cyt P450) significantly decreased after 10 days of PR administration. Similarly, the concentration of PBS was significantly decreased after 10 days of treatment with PR. On the other hand, these studies revealed a clear increase of mitotic activity and Ki-67 labelling index in the livers of animals treated with PR; furthermore, livers of PR-treated animals showed an increased percentage of binucleate hepatocytes. Flow cytometry analysis showed that although ploidy status of liver cells was not modified in any case the percentage of diploid nuclei in S-phase decreased during treatment with PR. The most relevant finding was the presence of abnormal mitosis and c-mitosis in livers from animals from all PR-treated groups. This study demonstrates that PR (a) does not induce cytotoxicity although it can induce cell proliferation and spindle disturbances in liver cells, (b) may also modulate the drug-metabolizing liver enzyme function, and (c) downregulates the expression of its own microsomal specific binding site.