The entrainment of circadian rhythms by light-dark (LD) cycles has been extensively investigated in laboratory studies. In almost all of these studies, organisms have not been allowed to modulate their exposure to the LD cycle. In the present study, the rhythm of running-wheel activity was investigated in nocturnal (domestic mice) and diurnal (Nile grass rats) rodents provided with light-tight nest boxes and maintained under long and short photoperiods. Photoperiod length had a significant effect on the duration of the daily active phase (alpha), on the phase angle of entrainment (psi), and on diurnality or nocturnality in both species. The availability of a nest box had a modest effect only on the variability of activity onsets. Neither in the nocturnal nor in the diurnal species was there any evidence of entrainment by frequency demultiplication or of entrainment without photic stimulation at either dawn or dusk. These results indicate that at least in the species studied, the ability of rodents to modulate their exposure to the LD cycle does not have a major effect on photic entrainment.