Orphanin FQ (OFQ), also known as nociceptin, is a member of the endogenous opioid peptide (EOP) family that has been functionally implicated in the control of pain, anxiety, circadian rhythms and neuroendocrine function. In the reproductive system, EOPs are involved in the steroid feedback control of gonadotropin releasing hormone (GnRH) pulses and the induction of the GnRH surge. The distribution of OFQ in the preoptic area and hypothalamus overlaps with GnRH, and in vitro evidence suggests that OFQ can inhibit GnRH secretion from hypothalamic fragments. Using the sheep as a model, we examined the potential anatomical colocalization between OFQ and GnRH using dual-label immunocytochemistry. Confocal microscopy revealed that approximately 93% of GnRH neurons, evenly distributed across brain regions, were also immunoreactive for OFQ. In addition, almost all GnRH fibers and terminals in the external zone of the median eminence, the site of neurosecretory release of GnRH, also colocalized OFQ. This high degree of colocalization suggested that OFQ might be functionally important in controlling reproductive endocrine events. We tested this possibility by examining the effects of intracerebroventricular (icv) administration of [Arg₁₄, Lys₁₅] OFQ, an agonist to the OFQ receptor, on pulsatile LH secretion. The agonist inhibited LH pulse frequency in both luteal phase and ovariectomized ewes and suppressed pulse amplitude in the latter. The results provide in vivo evidence supporting a role for OFQ in the control of GnRH secretion, and raise the possibility that it acts as part of an ultra-short, autocrine feedback loop controlling GnRH pulses.