The pituitary hormone prolactin is known to be produced in various extrapituitary tissues as well. We examined the presence of prolactin mRNA in the mouse gonads by reverse transcription (RT)-PCR and nucleotide sequence analysis. Both ovary and testis were found to express small amounts of mRNA coding prolactin, identical to that in the pituitary gland. Next, we established a sensitive competitive RT-PCR method to estimate the amount of prolactin mRNA and then measured its expression in the gonads during sexual maturation (10-80 days of age). In the ovary, the amount of prolactin mRNA (copies/microg of total RNA) gradually decreased from day 10 to 40, and the lower level was maintained until day 80. Little difference in the amount was observed between the estrous and diestrous groups on day 80. Conversely, the amount of testicular prolactin mRNA gradually increased from day 10 to 80. These results suggest that prolactin is produced in the mouse gonads and that it acts as an autocrine/paracrine factor modulating gonadal functions.