Five different physiological functions of the rabbit (hard faeces and urine excretion, food and water intake and locomotor activity) were registered during LD 12:12 and during continuous light conditions (LL). (1) In LD 12:12 a strong synchronization of the five parameters existed. The minima of all functions consistently occurred during the hours of light. The nocturnal percentage of overall 24-hr events was increased significantly in 'hard faeces excretion' (66 +/- 8(S.D.)%), 'water intake' (64 +/- 15(S.D.)%) and 'urine excretion' (58 +/- 10(S.D.)%). The nocturnal percentage of locomotor activity was significantly increased during the dark-hours in 9 out of 14 animals. In the other five individuals prominent peaks were present even during the photoperiod. On the average of all 14 animals 55 +/- 13(S.D.)% of the 24 hr events of locomotor activity occurred during the night. Despite a trough during the cessation of hard faeces excretion the events of food intake were not elevated significantly during the dark hours. (2) During LL the synchronization of the five functions within each animal persisted during the complete 90-day LL period. A free-running circadian rhythm with tau = 24.8 +/- 0.5(S.D.) hr was present in the four rabbits kept in LL conditions within 5-16 days after the withdrawal of the zeitgeber. (3) In addition to the circadian period the power spectrum analysis of data obtained during LD 12:12 revealed significant ultradian periods of an average period length of 11,6 hr (hard faeces and urine excretion), 8 hr (food and water intake, locomotor activity) and 4 hr (food intake, locomotor activity). During the free-run ultradian periods of 8 and 3.2-4.2 hr were significant in almost all parameters. (4) During LL the level of locomotor activity was reduced for 13 +/- 16(S.D.)%, the events of food intake were increased for 17 +/- 12(S.D.)%. (5) The reinserted LD 12:12 zeitgeber re-entrained the circadian rhythms within 25-45 days. (6) These results provided evidence of a predominant nocturnality of the rabbits under investigation.

Free-running circadian rhythms of rabbits were exposed to a 11:55-11:55-h light-dark (LD) schedule. After complete entrainment (63 +/- 22 days), the predominantly nocturnally active rabbits were exposed to an additional zeitgeber, restricted food access (RF), which was imposed during the light period. In five animals RF had the same period (T) as the LD cycle (23:50 h), and in five other animals TRF was 24:10 h. At a period of 23:50 h for both zeitgebers, the rhythms of four animals were stably entrained to RF, while in one animal a component of the rhythm broke away from RF and entrained to the LD zeitgeber. In animals exposed to zeitgebers of different periods most of the activity rhythm also entrained to RF, but 20 +/- 7% of the activity entrained to the LD zeitgeber. The light-entrained activity component merged with the RF component when the zeitgebers crossed, and decomposition occurred when the phase difference exceeded 4-6 h. The results indicate that two circadian oscillator systems exist in the rabbit, one entrained by light-dark cycles and the other by feeding-fasting cycles. Both exert common control over a number of overt behavioral rhythms.

Without a zeitgeber the circadian rhythms of five physiological functions free-ran with a period length greater than 24 h. Restricted feeding time (RF) masked the free-running rhythms. In addition to masking, entrainment with RF occurred. This process was most evident in locomotor activity and visits to the food box. RF thus had zeitgeber properties in these rabbits. However, in most rabbits the RF zeitgeber was not strong enough to entrain the circadian rhythm completely. A small component free-ran during RF. Following return to continuous food access the whole circadian rhythm resumed to free-run again. In some animals its phase was determined by the RF zeitgeber and in others by the small free-running fraction present during RF. The results suggest that in addition to the light-dark-entrainable circadian oscillator system a feeding-entrainable oscillator exists that takes over phase control of the majority of the rhythm during RF.

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