Free running activity and drinking rhythms of male Sprague-Dawley rats were observed in constant darkness (DD) for up to 44 days. The average period of the rhythms (tau DD) was 24.2 hr (+/- 0.12 hr) and the activity time was near one half of the circadian cycle. In the second experiment, rats were entrained to T cycles (T = period) with 2 hr of light per cycle. At T = 23 and T = 26 about one half of the rats entrained indicating that these periods are near the limits of entrainment. T = 23 induced a lasting aftereffect on tau DD while T = 26 affected tau DD only briefly. In contrast to some other nocturnal rodents, activity time was not compressed as T neared the limits of entrainment. In the third experiment, rats and hamsters were entrained to 24 hr skeleton photoperiods (two 1 hr light pulses/cycle). Rats phase jumped to the longer subjective night when the interval between the light pulses was reduced to 6 or 5 hr, while most hamsters phase jumped at 3.5 hr. Furthermore, all rats phase jumped by means of delaying transients while most hamsters showed advancing transients. Finally, while skeleton photoperiods compressed activity time in hamsters to 6 hr or less, activity time remained fairly constant in rats. These results demonstrate considerable differences in the organization of the circadian system among commonly studied nocturnal rodents.