Subcutaneous injections of monosodium L-glutamate (MSG) were administered, as a neurotoxin, at a dose of 4 mg/g body weight to rat pups on days 1 and 3 postnatally. Pinealectomy was performed at 6 weeks of age, and animals were allowed to mature. The intact MSG-treated rats showed a decrease of lordosis quotient in females and a reduction of ejaculation frequency and an increase in mount frequency in males. Pinealectomy caused no significant improvement on sexual behavior of the saline-treated normal rats, but it caused a marked improvement of the sexual receptivity in MSG-treated female rats and reinstated ejaculation in MSG-treated males. The circulatory sex steroid levels, estrogen in females and androgen in males, were not significantly different between the intact and pinealectomized MSG-treated groups in the respective sexes, whereas the progesterone level in females was significantly elevated in the pinealectomized MSG-treated group compared with the intact MSG-treated group. These findings suggest that the pineal gland plays an inhibitory role on the sexual behavior of MSG-treated rats and the improvement of sexual behavior following pinealectomy may be dependent on certain mechanisms other than the actions of circulatory androgens or estrogens.