Multiunit activity (MUA) of the suprachiasmatic nucleus (SCN) of the hypothalamus and medial preoptic area (MPOA) was recorded in chronically implanted, freely moving female rats. The integrated MUA of the SCN and MPOA was significantly higher in the dark than in the light period of day. Superimposed upon this diurnal rhythmicity in the SCN were 3.0- to 3.5-hour ultradian oscillations of MUA, with the fluctuations showing the highest variability at night. A significant increase in SCN MUA consistently preceded the offset of light by 1 h, whereas during the dark-light transition a marked decrease of neuronal firing occurred after the onset of light. Analysis of MUA base level – recorded during slow wave sleep – revealed that the average activity in the SCN was lowest on the day of diestrus-II, began to increase on proestrus night, and reached the highest values during estrus. Bilateral transection of the optic nerve, 4–6 weeks prior to electrical recordings, abolished the estrous cycle and the circadian pattern of neuronal firing of the SCN. An ultradian oscillation of integrated MUA in blind rats occurred with the same average intervals (3.0–3.28 h) but the amplitude was much higher than in intact cycling rats. Data indicate that there are correlated changes in basal MUA levels of the SCN and the stages of the estrous cycle. Furthermore, they suggest that maintenance of hormonal cyclicity and circadian rhythm of neuronal function requires intact retinohypothalamic conn