To investigate whether cell proliferation and PRL gene expression in female rat pituitary during estrous cycle were mediated by the poly (ADP)-ribosylation of chromatin proteins, anterior pituitaries at different estrous cycle were obtained from female Wistar rats, and poly (ADP-ribose) synthesis, DNA synthesis, PRL and GH messenger RNAs and PRL content in the pituitary, and serum concentrations of PRL and estradiol were analyzed. From diestrus to proestrus, poly (ADP-ribose) synthesis, the contents of PRL messenger RNA and PRL in the pituitary increased significantly, and decreased at estrus. However, DNA synthesis and serum concentration of PRL showed a significant increase from proestrus to estrus. Serum estradiol concentration increased from diestrus to proestrus. No significant change was observed in the pituitary GH messenger RNA content during estrous cycle. The increase of PRL messenger RNA from diestrus to proestrus was abolished completely by the administration of nicotinamide, an inhibitor of poly (ADP-ribose) synthesis, to rats at diestrus. These results indicate that poly (ADP)-ribosylation of chromatin proteins may play some role in cell proliferation and transcription of PRL gene during rat estrous cycle.