The tau mutation shortens the free-running circadian period of the activity rhythm of the Syrian hamster from around 24 h in the wild-type, to 20 h in the homozygous mutant. The aim of this study was to examine the effects of light pulses on the activity rhythms and expression of c-fos in the suprachiasmatic nuclei of wild-type and mutant hamsters. This would make it possible to determine the relative durations of subjective day and night, and thereby characterize further the effect of the mutation upon the circadian cycle. Presentation of light pulses (15 min) to animals kept in dim red light for seven to 10 days induced phase-dependent shifts in the onset of activity. During subjective day (inactive phase) there was no effect of light upon the activity rhythm of either normal or mutant animals. Early in subjective night (i.e. after the start of the active phase), light pulses induced phase delays of approximately one circadian hour in both phenotypes. However, later in subjective night light pulses induced phase advances in activity rhythms. The magnitude of the advance phase shifts differed considerably between the two phenotypes, with the wild-types typically showing shifts of about one circadian hour, while the mutants showed much larger advances of up to 12 circadian hours. In both strains, advances were observed following light at circadian time 22 but not after light at circadian time 24. The relative duration of subjective night was therefore comparable in the mutants and wild-types i.e. around 11 circadian hours.In order to examine the light-induced expression of the immediate early gene c-fos at different circadian phases, animals received a second light pulse (15 min) or a control dark pulse at the same circadian time as they had received the first, but seven to 10 circadian days later. They were perfused 1 h after the start of the pulse and brains processed for the immunocytochemical detection of Fos, the protein product of c-fos. Animals receiving dark pulses showed minimal Fos immunoreactivity in the suprachiasmatic nucleus, regardless of the circadian time of presentation of the stimulus. During subjective day (circadian times 4-11), light induced very little Fos-immunoreactivity. In contrast, light presented during subjective night (after circadian time 12) induced a dense pattern of Fos immunoreactivity in the ventrolateral, retino-recipient region of the suprachiasmatic nucleus of both wild-type and mutant animals. The phase of Fos-induction extended throughout subjective night and was closely linked both to activity onset and offset. During late subjective night/early subjective day (circadian times 24-3) there was a significant correlation between the timing of the light pulse relative to activity offset and the number of Fos immunoreactive nuclei in the suprachiasmatic nucleus. Overall, the relative duration of the Fos-inductive phase was not significantly different between wild-types and mutants.These findings suggest that the relative lengths of subjective day and night as defined by two criteria, behavioural phase-shifting and expression of c-fos are the same, in circadian hours, in the wild-type and mutant animals. This is consistent with the view that the tau mutation accelerates the circadian pacemaker, such that in the homozygote it consists of 24 circadian hours, each of only 50 solar min duration.