Rats were injected s.c. with monosodium glutamate (MSG); either a single injection of 2 mg/g body wt. was given on the 2nd day of life or repeated daily injections of the same dose were given from the 2nd to the 10th day. Controls received saline. The growth of repeatedly treated females was slightly retarded from day 80. Repeated treatment caused a slight reduction in endocrine organ weights in maturity; endocrine functions tested between 120-150 days of life were similar in control and treated animals, e.g., a.m.-p.m. difference in plasma corticosterone levels, adrenocortical stress responsiveness and glucose tolerance test (in males) and normal vaginal cyclicity, compensatory hypertrophy of ovaries and presence of pituitary castration cells after spaying (in females). None of these treatments influenced the sexual activity and fertility of either sex. Perinatal MSG treatment apparently does not cause hypopituitary syndrome in the mature rat.