A consistent group of gonadotropin-releasing hormone (GnRH) cell bodies occurs in the ventral hypothalamic tract at the infundibular lip (IL), just below the arcuate nucleus (ARC), at the site of the so-called GnRH 'pulse generator'. Immunocytochemical studies were performed to examine contacts between these GnRH neurons and nearby opioid peptide (OP) neurons in the ARC. Vibratome sections of the medial basal hypothalamus were obtained from colchicine-treated, perfusion-fixed juvenile female rhesus macaques. They were sequentially immunostained for GnRH using the peroxidase antiperoxidase (PAP) technique and for adrenocorticotropic hormone (to identify OP neurons) using colloidal gold. The PAP and colloidal gold markers could be clearly differentiated at both the light and electron microscopic levels. OP+ and GnRH+ neuronal cell bodies occurred close together in the ARC-IL region, sometimes within the same electron microscope grid square. At the electron microscopic level, OP+ axons formed symmetrical synapses with GnRH+ somata and proximal axons, suggesting a pronounced inhibitory influence on GnRH neuronal activity. Examples of OP+/GnRH+ axodendritic and dendrodendritic contacts were also observed. Furthermore, symmetrical synapses between GnRH+ axons and GnRH+ perikarya or dendrites were occasionally present. The data obtained here clearly indicate that direct OP inhibition of GnRH 'pulse generator' neurons occurs at the ARC-IL in juvenile primates. It is suggested that these OP neurons help mediate steroid-negative feedback at the hypothalamic level. Furthermore, it is suggested that OP/GnRH and GnRH/GnRH inhibitory contacts may play a role in maturation and control of reproductive function.