In this immunocytochemical study, heterogeneity in the storage of gonadotropins in the sheep fetus was investigated. LH cells were first detected at 50 days of gestation, whereas FSH and LH-FSH cells were observed from Day 89 on; no gonadotropes containing ACTH were observed. LH cell percentage increased for both sexes between 99 and 120 days of gestation (from 2.8 +/- 0.1 to 4.0 +/- 0.3), remained high until term, and did not change after birth. In males, FSH cell percentages increased about twofold between 120 and 130 days to 4.0 +/- 0.4, fell at 140 days to 1.6 +/- 0.1, and then increased in the newborn lambs about twofold. A similar pattern of development was observed in females, but the percentages were lower than in males. LH-FSH cell percentages increased about twofold in males between 99 and 120 days of gestation to 1.1 +/- 0.1, decreased to 0.8 +/- 0.1 at 140 days, then increased about twofold in newborn lambs. In the 99-day-old females, this percentage was 0.5 +/- 0.1; it remained stable between 99 days of fetal life and term and did not change after birth. Hence, the pattern of change in the percentages of these cells appeared to be similar to that of LH cells and different from that of FSH cells. This suggests that LH-FSH cells might result from a shift of monohormonal cells to bihormonal cells. However, these cells seem to be regulated differently in male and female fetuses, and this suggests that they possess a specific role during ontogenesis.