The circadian caecotrophy rhythm of the rabbit was monitored in LD 12:12 and continuous light of two different intensities.1.The length of the free‐running rhythm and the phaseangleof theen trained rhythm were found to be influenced by after‐effects for a considerable time span of up to 75 days.2.The free running period in LL of 1200lux was significantly longer (24.94 ± 0.71(SD)h) than in 10 lux (24.23 ± 0.39 (SD) h).3.In 10 lux the caecotrophy rhythm ran free monophasically, even in those rabbits practising caecotrophy twice in LD 12:12. This confirms initial observations made in 300 lux. In LL of 12001ux, “splitting"ocurredin 4 out of 8 animals.4.The entrainment following re‐insertion of the LD 12:12 after LL depended on the strength of the Zeitgeber and the phase of the rhythm at the end of LL.5.A correlation existed between the phase angle difference and the length of the spontaneous period: the shorter the spontaneous period, the less negative was the p.a.d.6.The results suggest that the caecotrophy rhythm of the rabbit accords in several chronobiological characteristics to the activity rhythm of nocturnal rodents.