The amount of substance P-immunoreactivity (SP-I) was studied in the female rat reproductive organs (uterus, cervix, vagina and ovary) in the diestrous and estrous stages of the estrous cycle, after ovariectomy and on days 7, 14 and 19 of pregnancy. The concentration of SP-I (pmol X g-1 tissue) was generally highest in the vagina followed by cervix, uterus and ovary. SP-I concentrations in all reproductive organs were significantly higher following ovariectomy but significantly lower during pregnancy compared to concentrations from intact rats. There were no significant differences in the total content of SP-I per organ during the estrous cycle, after ovariectomy or during pregnancy (cervix, vagina). SP-I concentrations were markedly reduced in the reproductive organs of adult rats treated with capsaicin as neonates. This led to the conclusion that SP in reproductive organs is present in primary afferent fibers. The findings indicate that the number of SP-I containing primary afferent fibers in the female reproductive organs are stable elements neither increasing nor decreasing in response to changing hormonal levels or to the tissue growth and differentiation occurring during the estrous cycle or pregnancy.