1. Fourteen experiments were performed to determine the minimum photoperiod and minimum scotoperiod needed for entrainment of oviposition in hens exposed to 21-h, 24-h and 30-h cycles. Entrainment was measured by the proportion of total eggs laid in a modal 8-h segment of each cycle. 2. In a 24-h light and dark cycle, a 15-min photoperiod or a 5-h scotoperiod produced essentially the same degree of entrainment as 6 h light, 18 h darkness (6L : 18D) or 14L : 10D, which were used as control treatments. Under 21-h light and dark cycles a minimum 3-h photoperiod or a minimum 9-h scotoperiod was needed to achieve full phase setting. When the cycle length was increased to 30 h a minimum 8-h photoperiod or a minimum 12-h scotoperiod was required for full entrainment. 3. This study demonstrates that photoperiod is a stronger signal than scotoperiod for the purpose of phase setting oviposition. It also shows that a stronger signal is required to achieve entrainment when the length of the light-dark cycle is several hours shorter or longer than the natural period of 24 h.

1. Nine short trial, involving 96 different treatments, were used to investigate the critical intensities and duration of bright and dim periods of lighting needed to entrain oviposition in cycles ranging from 21 to 30 h. 2. Entrainment was shown to depend upon the contrast between bright and dim lighting, and to be independent of the absolute light intensity. 3. A bright: dim ratio of 13:1 fully entrained oviposition in cycles of 25 h and 27 h. For 23-h and 28-h cycles a 30:1 ratio was required. Twenty-one-hour cycles required a ratio of 300:1 and with 30-h cycles a ratio of 1000:1 was needed to achieve full entrainment of oviposition. 4. In 24-h cycles, 1 h of bright lighting at 02.00 h was sufficient to override other environmental signals and cause eggs to be laid in the late evening, but a minimum bright period of 6 h was needed to cause full phase setting with 21-h cycles. 5. Circadian periodicity can easily be imposed on hens by providing a short exposure to bright light with a background of continuous dim light; but the signal must be increased (by providing a greater contrast between bright and dim lights and/or a longer period of bright lighting) to entrain oviposition when the cycle deviates markedly from the natural period of 24 h.

1. A series of four "sunset" signals, without associated sunrises, was given to laying hens by reducing light intensity in steps from 190 lx to 63, 21, 7 and 2 lx at 24-h intervals. Four "sunrises" were given by reversing the process. Treatments involving single "sunrises", single "sunsets" and combinations of sunrises and sunsets at 12-h intervals were also tested. 2. Only treatments which included one or more sunset signals were found to give full entrainment of oviposition, as judged by the proportion of eggs laid in a modal 8-h segment of each 24-h cycle. Full entrainment was achieved if the sunset occurred between 16.00 h and midnight, but not if the sunset occurred in the morning hours. Treatments involving only sunrise signals did not fully entrain oviposition but did have effect on the timing and distribution of ovipositions. Sunrises given in association with sunsets increased the degree of entrainment achieved, as compared with the sunset signal alone. 3. This study indicates that sunset is a potent synchronising agent for phase setting oviposition and, presumably, for phase setting the endogenous circadian rhythm of the fowl. Sunrise can also exert a modifying influence.

Under continuous lighting, time of oviposition was controlled by a temperature cycle (12 h at 30 degrees C and 12 h at 20 degrees C). The peak of egg laying occurred 15 h after the beginning of the cool period, which is the same interval as the interval observed between lights-out and peak egg laying under a 12L:12D lighting schedule. 2. When the temperature cycle was set 12 h out of phase with a light-dark cycle (12L:12D; temperature reduced when the lights came on), oviposition was entrained by the light-dark cycle. 3. When a temperature cycle was set 6 h in advance or 6 h in arrears of a 12L:12D cycle there was a difference of 1-4 h in mean time of lay between the two treatments, indicating that temperature can have a significant, though subsidiary, effect on oviposition time in the presence of a clear light-dark signal. 4. When the light-dark signal was reduced to 22L:2D, oviposition time was controlled by the temperature cycle. With 20L:4D oviposition time was determined principally by the photoperiod, but with a subsidiary effect due to temperature.

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