The effect of the pineal gland on prolactin (PRL) synthesis, storage and release was tested in female rats. To do this, we incubated in vitro hemi-anterior pituitaries from 80-day-old female rats that had been rendered, 8 weeks previously, either blind-anosmic, blind-anosmic and pinealectomized or left intact. Reproductive organ weights, pituitary weights and pituitary DNA content were decreased in animals rendered both blind and anosmic. These effects were reversed by removal of the pineal gland. Additionally, the serum levels of PRL were diminished in blind-anosmic rats from that of intact controls. The synthesis of PRL in vitro was dramatically reduced in the pituitaries of blind-anosmic rats as evidenced by a 63% decrease in [3H]leucine incorporation into PRL from that observed in the intact group. Likewise, the total amount (medium + pituitaries) of radioimmunoassayable PRL in vitro was depressed by 26% in the blind-anosmic group as compared with intact controls. Pinealectomy reversed the reductions in de novo synthesized and total immunoreactive PRL in vitro. From these studies we conclude that in the blind-anosmic female rat the pineal chronically inhibits PRL synthesis and storage in the pituitary and, possibly, its release into the blood. These pineal-induced effects could be accounted for by a reduction in the pituitary mammotroph as might be indicated by the decrease in pituitary DNA content observed in dual-sensory deprived rats.