EEG recordings of hippocampal theta activity and neocortical activity were correlated with direct observations and photographic recordings of the behaviour of unrestrained cats. The behaviour studied included (1) both spontaneous and induced changes in level of arousal (sleep-wakefulness), (2) different types of orienting activity that occur as the animal is repeatedly exposed to a novel environment, and (3) activities such as walking, drinking, and grooming. With increases in arousal, amount of hippocampal theta activity increased in direct association with extent of pupillary dilatation, degree of eye opening, and extent of neocortical desynchronization. Conversely, with spontaneous or habituation-induced reductions of arousal, amount of hippocampal theta activity became reduced. Maximal hippocampal theta activity was associated with the increased eye movement phase of REM sleep and, during wakefulness, with fixed staring and visual searching behaviour ('tonic' orienting activity). In contrast, hippocampal theta activity was suppressed or attenuated during reflexive head turning ('phasic' orienting behaviour) and other stereotyped movements such as drinking, grooming and undirected locomotor activity unaccompanied by attentive behaviour. These results suggest that theta activity is primarily related to arousal and its control by attentional mechanisms.