The suprachiasmatic nuclei (SCN) of the hypothalamus continue to oscillate when they are isolated in a brain slice preparation. We recorded multiunit activity in the SCN of the rat both in vivo and in vitro to determine the circadian discharge pattern. The variability of the discharge pattern is larger and the amplitude of the rhythm is smaller in vivo than in vitro. Moreover we found evidence for a direct effect of the animal's behavioural activity on electrical activity of the SCN in vivo. These findings may provide an electrophysiological basis for the known effects of behavioural stimuli on the circadian pacemaker. This study underscores the importance of recordings in intact preparations in addition to in vitro work when generalisations to physiological conditions are to be made.