The suprachiasmatic nucleus (SCN) of the hypothalamus has an essential role in orchestrating circadian rhythms of behaviour and physiology. In the present study, we recorded from single SCN neurons in urethane-anesthetised rats (male, 250-450g) using the ventral surgical approach, categorized them by statistical features of their electrical activity and by their responses to light, and examined how activity in the light phase differs from activity in the dark phase. We classified cells as light-on cells or light-off cells according to how their firing rate was acute response to light, or as non-responsive cells. In both sets of light-responsive neurons, responses to light were stronger at subjective night than in subjective day (mean difference of light on cells or light off cells evoked by light; 4.1±0.6 and 1.8±0.4 spikes/s or -1.7±0.4 and +0.2±0.4 spikes/, each). Neuronal firing patterns were analysed by constructing hazard functions from interspike interval data. For most light-responsive cells, the hazard functions showed a multimodal distribution, with a harmonic sequence of modes, indicating that their spike activity was driven by an oscillatory input with a fundamental frequency of close to 30Hz; this harmonic pattern was very rarely seen in non-responsive SCN cells. The frequency of the rhythm was the same in light-on cells as in light-off cells, the same in subjective day as at subjective night, and was unaffected by exposure to light. Paired recordings indicated that the discharge of adjacent light-responsive neurons was very tightly synchronized, consistent with electrical coupling. Whether the activity of light responsive cells in the SCN is globally synchronised, giving rise to a coherent 30Hz output the amplitude of which is modulated by light intensity, remains to be determined; it is possible that there is only a localised synchronisation with multiple subpopulations discharging out of phase with each other. As both light-on cells and light-off cells discharge in rhythms locked to a 30Hz cycle, an interesting possible is that interactions between these populations mean that their discharge is out of phase (1).