The ovulatory process in mammals involves gross physiological events in the ovary that cause transient deterioration of the ovarian connective tissue and rupture of the apical walls of mature follicles. This gonadotropin-induced process has features similar to an acute inflammatory reaction that affects most of the ovary. The present study reveals that the ovulatory events include induction of mRNA for pancreatitis-associated protein-III (PAP-III). Immature Wistar rats were primed with 10 IU equine chorionic gonadotropin s.c., and 48 h later the 12-h ovulatory process was initiated by 10 IU human chorionic gonadotropin (hCG) s.c. Ovarian RNA was extracted at 0, 2, 4, 8, 12 and 24 h after the animals were injected with hCG. The RNA extracts were used for RT-PCR differential display to detect PAP-III gene expression in the stimulated ovarian tissue. Northern blotting showed that transcription was significantly greater at 4-12 h after the ovaries had been stimulated by hCG. In situ hybridization indicated that PAP-III mRNA expression was limited mainly to the hilar region of the ovarian stroma, with most of the signal emanating from endothelial cells that lined the inner walls of blood vessels, and from small secondary follicles. Treatment of the animals with ovulation-blocking doses of indomethacin (an inhibitor of prostanoid synthesis) or epostane (an inhibitor of progesterone synthesis) revealed that ovarian transcription of PAP-III mRNA was moderately dependent on ovarian progesterone synthesis. In conclusion, the present evidence of an increase in PAP-III gene expression in gonadotropin-stimulated ovaries provides further evidence that the ovulatory process is comparable to an inflammatory reaction.