The suprachiasmatic nucleus (SCN) of the hypothalamus has an essential role in orchestrating circadian rhythms of behavior and physiology. In the present study, we categorized SCN neurons by statistical features of their electrical activity, and by their responses to light, and examined how recordings made in the light phase differ from recordings made in the dark phase. Male Sprague-Dawley rats (250-450 g) anesthesized by urethane (ethyl carbamate, 1.3 g/kg i.p.) were tracheotomized and the optic chiasm just below the SCN region was exposed by ventral surgery (Leng & Dyball, 1991). We recorded from 671 SCN cells (375 cells, mean rate 7.30 ± 0.34 Hz in rats maintained on a normal light cycle, 296 cells, mean rate 10.1 ± 0.47 Hz in rats on a reversed light cycle). We subdivided cells into three groups according to their light-responsiveness: light-on cells that increased their firing frequency in response to light; light-off cells that decreased their firing frequency; and non-responsive cells. In rats that had been maintained on a reversed light cycle, light-on cells fired at a higher mean rate than in rats that had been maintained on a normal light cycle, and their responsiveness to light was stronger. Neuronal firing patterns in conditions of maintained room light on and room light off were analysed by constructing hazard functions from interspike interval data (Leng et al. 1995; these functions display how the excitability of a cell changes with time since the last spike). For most light-responsive cells, the hazard functions showed a multimodal distribution, with a harmonic sequence of modes, indicating that the neuronal discharge was driven by an oscillatory input; this oscillatory pattern was rarely seen in non-responsive SCN cells. The “oscillatory” cells comprised two distinct populations: about 40% of the light-on cells (46 in normal light cycle and 53 in reversed light cycle) fired with a first mode at 33.6 ± 0.8 ms reflecting an oscillatory drive at ~ 30 Hz, and about 60% (74 in normal light cycle and 73 in reversed light cycle) fired with a first mode at 88.7 ± 4.2 ms reflecting an oscillatory drive at ~ 15 Hz. By contrast, only about of 10% of light-off cells (2 in normal light cycle and 9 in reversed light cycle), fired with an early first mode compared to 90% (48 in normal light cycle and 45 in reversed light cycle) with a late first mode. These data suggest that light-responsive cells are organized into local networks that generate strong rhythms of activity, that these rhythms differ between light-off cells and light-on cells, and that these rhythms differ according to the stage of the light cycle.