Activity data from two experimental lizard groups were analysed in order to search for 24 hentrained and ultradian periodicities. The data of a first group were obtained through motion sensitive platforms situated under the animals' cage and continuously for up to 12 consecutive days; those from a second group were collected by manual recording of the behaviour patterns of individual animals for 2 h each day over a 10-day period. Lizards from both groups were situated in cages inside isolated chambers in which a light-dark cycle (12:12), a temperature of 28°C ±1° and a relative humidity of 50-60% were maintained. Periodogram analysis showed the existence of a significant period peak (p < 0.01) at 24 h. Autocorrelation functions and spectral analysis of different data-segment lengths showed that ultradian periodicities were present in the daily motor activity, appearing as noisy though frequency-band limited. Differences in the frequency bandlimited fluctuations were found between morning and afternoon activities: during the morning ultradian activity appears distributed in two bands (4.5-36 c/day and 63-94 c/day), the power being mainly concentrated in the second one, while during the afternoon it was in the 4.5-36 c/day band. Results from the second lizard group showed only one band (24-60 c/day) overlapped with the first one from the latter group. Although activity recording methods and some experimental conditions are discussed as possible sources of these differences, the possibility of endogenous ultradian variation within the individuals is also suggested.