Neuropeptide Y (NPY) is present in large quantities in the hypophysiotropic areas of the brain and has a similar distribution pattern as gonadotropin-releasing hormone (GnRH). The aim of this study was to measure GnRH release during perfusion of NPY through the mediobasal hypothalamus of intact and ovariectomized (OVEX) does. Does were fitted with push-pull (PP) cannulae directed into the median eminence and were subjected to subsequent PP perfusion at a flow rate of 20 µl/min for a total of 6 or 8 h. Six intact does and seven OVEX does received intrahypothalamic perfusion of NPY (14 µg/ml) for either 2 or 3.5 h during the 6 or 8 h, respectively, of PP perfusion. The PP samples and femoral vein blood samples were collected at 10-min intervals on ice. Perfusate GnRH and plasma luteinizing hormone (LH), follicle-stimulating hormone (FSH), and prolactin (PRL) levels were measured by specific radioimmunoassays. The pulsatile parameters of all hormones were analyzed by the ‘PULSAR’ program. Perfusion of NPY significantly decreased mean levels, pulse amplitudes, and pulse frequencies of GnRH in OVEX does. Also, mean levels of LH were decreased, whereas levels of FSH and PRL were unaffected by NPY. In contrast to these inhibitory effects on GnRH in OVEX does, the same NPY perfusion stimulated mean levels and pulse amplitudes of GnRH in intact does. Plasma levels of neither gonadotropin nor PRL were affected significantly by the NPY treatment in intact rabbits. These results indicate that NPY may have central effects on neuronal release of GnRH, and that ovarian factors are critical in the directional mode of these NPY ac