We reported previously that pancreatic-type group I phospholipase-A2 (PLA2-I) stimulates DNA synthesis or prostaglandin E2 production in various cell types via its specific receptors on the cell surface. In the present study we examined the effect of PLA2-I on corpus luteum, which possesses specific PLA2-I receptors, and demonstrated gene expression of PLA2-I in rat ovaries. PLA2-I stimulated progesterone release from incubated corpora lutea at concentrations above 10 nM in a dose-dependent manner. Northern blot analysis revealed the presence of PLA2-I messenger RNA (mRNA) in rat ovaries. The level of PLA2-I mRNA was elevated during diestrus to proestrus, and decreased to almost nothing on the day of estrus. To further evaluate the timing of PLA2-I expression, we analyzed RNAs extracted from small follicles, large preovulatory follicles, and corpora lutea obtained from various phases of hCG-treated immature rats. During follicular development, mRNA levels remained negligible. The level of PLA2-I mRNA began to rise after luteinization of follicles and increased during the maturation of the corpora lutea. Levels of PLA2-I mRNA in ovaries from day 5 and day 10 pregnant rats were 5- to 10-fold higher than those in diestrous rats, whereas on day 21, the mRNA level was decreased. Immunocytochemical studies were performed to clarify the synthesis and localization of PLA2-I in the ovary using polyclonal anti-PLA2-I antibodies. Intense immunoreactivity was detected only in luteal cells of newly formed corpora lutea. However, the number and intensity of immunoreactive cells decreased in old corpora lutea. No immunoreactivity was detected in follicular cells of small- or middle-sized follicles. These results demonstrate the correlation among PLA2-I gene expression, estrous cycle, and progesterone secretion in rat ovaries and suggest a new function of PLA2-I as an intragonadal regulatory factor for steroidogenesis.