The present study was concerned with the effects of a transplantable prolactin-secreting pituitary tumor (7315b) on the hypothalamic release of dopamine, luteinizing hormone-releasing hormone (LHRH) and thyrotropin-releasing hormone (TRH) in gonadectomized, adrenalectomized male rats bearing subcutaneously a testosterone capsule and a corticosterone pellet. Similar male rats not inoculated with tumor served as controls. The rats were studied 3–4 weeks after tumor inoculation, while they were anesthetized with urethane. Compared to the controls, prolactin levels in the tumor-bearing rats had increased 70-fold, whereas the levels of luteinizing hormone (LH) and follicle-stimulating hormone (FSH) decreased to 20 and 27%, respectively. In tumor-bearing rats, the secretion of dopamine into hypophysial stalk plasma increased from 2.3 to 4.9 ng/h (p < 0.025), whereas that of LHRH decreased from 127 to 52 ph/h (p < 0.005). Since the use of urethane anesthesia may change quantitatively and qualitatively the effects of hyperprolactinemia, it was decided to study these effects on the in vivo release of LHRH, dopamine and TRH in conscious rats by a push-pull perfusion of the median eminence-arcuate nucleus area. Using this technique, it was found that in tumor-bearing rats the secretion of LHRH decreased from 20.0 to 9.8 pg/15 min·(p < 0.005), whereas that of dopamine increased from 118 to 246 pg/15 min (p < 0.025). The secretion of TRH was not altered by hyperprolactinemia (4.1 vs. 4.4pg/15 min). Thus, push-pull perfusion experiments in conscious, freely moving rats confirm what was found in hypophysial stalk blood obtained from rats under anesthesia, namely, that hyperprolactinemia stimulates dopamine release and inhibits LHRH release. This study also indicates the value of using conscious rats to determine simultaneously the in vivo output of several substances from the hypothalam