The prion protein (PrP) is a glycoprotein anchored to cell membranes and expressed in most cell types. Its structural features indicate possible relations to signal peptidases (Glockshuber et al. 1998). Since mutations in this protein lead to severe neurodegeneration and death in humans and animals, it is possible that the loss of its normal function contributes to the development of the pathology. Little is known about its normal function, but there are indications that it may play a role in circadian rhythm and sleep regulation in mice. We explored further whether PrP plays a role in sleep regulation by comparing sleep and the effects of 6 h sleep deprivation in PrP knockout mice and isogenic wild-type mice of the 129/Ola strain. The mice did not differ in the amount and distribution of the vigilance states or in the power spectra. The most remarkable difference was the larger and long-lasting increase of slow-wave activity (mean EEG power density 0.75–4.0 Hz) in non-rapid-eye-movement (NREM) sleep during recovery from sleep deprivation in the null mice. The results confirm our previous findings in mice with a mixed background. This observation applies also to slow-wave activity in NREM sleep episodes following spontaneous waking bouts of different duration. Sleep fragmentation in both genotypes was larger than in mice with the mixed background. A new aspect was revealed by the spectral analysis of the EEG, where the null mice had a lower peak frequency within the theta band in REM sleep and waking, and not in NREM sleep. Behavioural observations concomitant with the EEG indicated that the EEG difference in waking may be attributed to the smaller amount of exploratory behaviour in the null mice. The difference between the genotypes in theta peak frequency was not an overall effect on the EEG, since it was absent in NREM sleep. PrP therefore may be affecting the theta-generating mechanisms in the hippocampus during waking and REM sleep. It remains unresolved whether PrP plays a role in sleep consolidation, nevertheless the data suggest that it is involved in sleep regulation. A passive avoidance test showed a difference between the genotypes. It is not probable that this was due to memory differences, since the genotypes reacted similarly in a delayed T-maze alternation procedure. The behavioural differences need to be pursued further.