In the present study two recently developed techniques have been combined to enable the simultaneous in vivo determination of pulsatile release of GnRH, LH, and FSH in the orchidectomized rat. The first of these techniques involves the implantation of two vascular catheters and collecting serial blood samples through one while simultaneously infusing a replacement blood mixture through the other; consequently, blood samples can be collected for an extended period of time, and detailed plasma LH and FSH release profiles can be established for individual animals. The second technique involves push-pull perfusion of the pituitary gland to determine changes in GnRH concentration as might be perceived by the gonadotropes. For each animal (n = 6), blood (150 microliters) and push-pull perfusate (200 microliters) samples were collected at 5- and 10-min intervals, respectively, for approximately 6 h, and the hormone release profiles were determined by RIA. All of the rats showed a clear pulsatile release pattern for GnRH, LH, and FSH. Moreover, the interpulse interval was remarkably similar for each of these hormones (36.9, 41.5, and 43.5 min, respectively, as determined by PULSAR). The percentage of GnRH pulses associated with a gonadotropin pulse was 72% for LH and 76% for FSH; only 14% of the pulses were silent for both gonadotropins. These results demonstrate that in the orchidectomized rat the pulsatile pattern of GnRH release is reflected in the pulsatile pattern of not only LH but also FSH. They may, therefore, be construed to support the concept that the pulsatile secretion of both gonadotropins is primarily orchestrated by a single hypothalamic releasing hormone. Alternatively, if two separate hypothalamic releasing hormones do indeed exist (LHRH and FSH-releasing hormone), it would appear that in the orchidectomized rat their episodic release is tightly coupled to the same hypothalamic pulse generator.