Experiments were carried out to investigate the relationship between levels of arousal and the temporal discharge pattern of hypothalamic neurons in unanaesthetized, unrestrained rats and rabbits. Extracellular recordings were taken from 22 hypothalamic neurons in animals that had been implanted previously with platinum microwire electrodes. Separate records of neuronal activity were taken from each neuron when the animal was at two extremes of arousal, sleep and alarm, and compared with an intermediate state of arousal, awake and relaxed. The extremes of arousal were defined by simple behavioural criteria, shown to coincide with specific patterns of electrocorticographic activity. Interval distributions were constructed from these records of neuronal activity. The modal interval, but not the frequency of discharge of these neurons, changed in a consistent manner with the level of arousal for all the neurons recorded. The modal interval was always short (9.59 +/- 1.2 ms (mean +/- s.e.), n = 17) during sleep and longer when the animal was alarmed (57.15 +/- 7.59 ms, n = 13). When the animals were awake and relaxed the modal interval was between those of sleep and alarm (27.5 +/- 2.79 ms, n = 19). Scatter about an individual mode was greater in sleep than during alarm. It is suggested that the continuum of arousal from sleep to alarm is reflected by a continuously shifting modal interval for each hypothalamic neuron. This is essentially similar to reports on the effect of arousal on cortical neurons.