Wheel-running behaviour of the Nile grass rat, Arvicanthis niloticus, was studied under a variety of lighting conditions to characterize circadian rhythms in this species. A series of lighting schedules was used to determine the nature of entrainment, the rates of reentrainment after 6-h phase shifts, and the stability of free-running rhythms in constant light (LL) and constant dark (DD). All 15 individuals showed peaks of activity around dawn and dusk, 11 were more likely to run when lights were on and 4 were more likely to run when the lights were off. Three of the latter animals were completely crepuscular, but 1 showed a distinctly different and relatively nocturnal pattern, remaining active well into the dark phase. All 7 animals that were exposed to 6-h phase advances and delays reentrained to both shifts within 9 days. Stable and precise free-running rhythms were exhibited by all animals in LL and DD, and periods were longer in LL than DD. The ratio of the active to the inactive phase was significantly higher in LL than in DD or LD conditions. We conclude that A. niloticus exhibit predominantly diurnal running rhythms, with peaks of activity occurring around dawn and dusk, and that their rhythms are stable and precise and respond rapidly and predictably to changes in lighting conditions.