Follicle-stimulating hormone (FSH) secretion is increased in the immature female rat from day 5 to days 17-18 of life, and decreases steadily thereafter until puberty. It has been reported that estradiol negative feedback and inhibin-like peptides are low during this period, while luteinizing hormone (LH) and FSH sensitivity to LH-releasing hormone (LHRH) are maximal. It was therefore of interest to study the effects of some neurotropic drugs on FSH release at 12 days of age, and to compare their effects at 1 and 20 days. Besides, as developmental patterns and regulation of FSH are different in male and female rats, the experiments were carried out using male and female littermates. The drugs chosen were haloperidol, 5-hydroxytryptophan and naloxone. These drugs release LH in the infantile female rat, the effect decreasing or disappearing as the animal matures; no effects of these drugs have been reported on FSH release in infantile rats to the present time. It was found that haloperidol (0.25 mg/kg), naloxone (2 mg/kg) and 5-hydroxytryptophan (50 mg/kg) markedly increased the already high titers of FSH in the 12-day-old female rat. This effect could not be discerned in newborn rats, and had disappeared at 20 days of age. Male littermates failed to respond at any age. When adult male and female rats in diestrus were tested, all drugs at the chosen doses were ineffective in altering FSH release. These data suggest that the infantile female rat represents an interesting physiological model to evaluate the neural regulation of FSH in a situation in which inhibitory signals provided by inhibin and estrogen in later life are diminished.