The relative importance of sunrise and sunset for entrainment of oviposition in the fowl.

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.