This study investigates the effects of intraventricularly administered gonadotropin-releasing hormone (GnRH) on plasma beta-endorphin levels in female proestrous rats. Adult female Wistar rats (220-250 g) were implanted with an indwelling cannula in the third ventricle. Approximately 20 days later, the animals which had established a regular 4-day cycle were implanted with two indwelling catheters, one intracarotid and one intrajugular, on the morning of proestrus. A single injection of 100 ng GnRH dissolved in 5 microliters distilled water or 5 microliters of saline (control) was infused slowly through the cannula in the third ventricle. Blood was withdrawn via the intracarotid catheter just before the infusion (12.30 h) and at 14.00, 15.30, 16.30 and 17.30 h for the determination of plasma beta-endorphin levels. The results indicated that intracerebroventricular infusion of GnRH causes a significant decline of plasma beta-endorphin levels at all time points. It is postulated that GnRH possibly causes desensitization of GnRH receptors, due to the continuous GnRH supply to the pituitary via the blood circulation.