Relatively little is known regarding the potential importance of LHRH and of gonadotropin-releasing hormone-associated peptide (GAP) on the postnatal development of gonadotrophs. We investigated the effects of administration of anti (A)-LHRH serum or A-GAP serum to neonatal rats on the development of LH and FSH immunoreactivity in anterior pituitary gland (APG) cells. Serum (sheep non-immune [NSS], sheep A-LHRH, sheep A-LHRH/GAP [which bound LHRH and GAP], rabbit non-immune [NRS], or rabbit A-GAP) was injected s.c. into neonatal female and male rats on Days 1 and 3 or Days 1, 3, 5, and 7 after birth. Pups were killed on Day 5 or 9, two days after the last injection. The percentages of APG cells immunoreactive for LH or FSH increased from Day 1 to Day 5 and did not change between Days 5 and 9 in female pups treated with NSS or NRS. There was a trend for the percentages of LH and FSH cells to increase from Day 1 to Days 5 and 9 in male pups treated with NSS or NRS, but the increases were not statistically significant. In both females and males, treatment with antisera that recognized LHRH reduced the percentage of FSH cells on Day 5 and the percentages of LH and FSH cells on Day 9. Treatment with A-GAP was without effect in both sexes. There were similar percentages of LH and FSH cells in females and a lower percentage of FSH than of LH cells in males in the Day 5 and 9 controls.(ABSTRACT TRUNCATED AT 250 WORDS)