The effects of hyperprolactinemia (hyperPRL) and hypoprolactinemia (hypoPRL) on pituitary gonadotropin secretion and the feedback sensitivity to testosterone (T) were evaluated in immature male rats. At 34 days of age, rats were divided into three groups: group 1, controls, injected with oil; group 2, treated with bromocriptine mesylate (CB-154; 250 micrograms in oil/rat X day); and group 3, subjected to the transplantation of one pituitary from an adult female rat under the kidney capsule and treated with oil. The treatments were continued for 14 days. On day 8, each of these groups were further divided into three subgroups: intact, castrated, and castrated plus T treated. T treatment consisted of injection of T propionate (TP; 50 micrograms in oil/rat) on alternate days starting immediately after castration. Blood samples were obtained by cardiac puncture throughout the study. Plasma PRL levels were significantly reduced by CB-154 treatment and significantly increased by the pituitary graft (P less than 0.001). In intact immature male rats, hyper- or hypoPRL did not affect plasma LH levels, whereas hyperPRL reduced (P less than 0.01) plasma FSH concentrations. The postcastration increase in circulating LH levels was significantly increased (P less than 0.001) in rats treated with CB-154 24 h after castration. Moreover, the suppressive effects of TP on plasma LH and FSH levels were significantly (P less than 0.05) attenuated in hypoPRL rats. In pituitary-grafted rats, effects of castration and TP replacement on plasma LH levels did not differ from those in control rats. These results demonstrate that subnormal levels of PRL reduce the sensitivity of the hypothalamic-pituitary system to feedback inhibition by T. In contrast to previous findings in the adult rats, acute hyperPRL in immature male rats did not affect the negative feedback action of T on gonadotropin secretion.