Inhibition of wheel running by light (masking) was investigated in Syrian hamsters with suprachiasmatic nucleus or sham lesions. Approximately 90% of the wheel revolutions made by hamsters with complete suprachiasmatic nucleus lesions, as judged by histology and power spectrum analysis of their wheel running, occurred during the dark phases of an ultradian light-dark cycle (3.5 h light, 3.5 h dark). This was demonstrated for two illumination levels (380 lx and 6 lx). Similar results were obtained with sham-operated animals. In further tests, the hamsters with lesions also retained a strong preference for the dark side of a box divided into dark and light sides. These results demonstrate that the suprachiasmatic nucleus is not necessary for masking by light or the preference for a dark over a light compartment. Evidently the direct effects of light can substitute for the endogenous control by the suprachiasmatic nucleus to maintain appropriate behaviour in time and space.