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Partial synchronization of oestrous cycles was observed in Sprague–Dawley rats with both 4 and 5 day cycles. In groups of rats exhibiting a synchrony of cycles, the number of animals that came into oestrus 4 or 5 days (approximately one cycle) after they were placed in a new environment was significantly greater than the number expected to be in oestrus if cycling was random. An environmental influence was considered to be responsible for the synchrony of the oestrous cycles but a specific stimulus for their synchronization was not identified. Initiation of vaginal smearing was considered to be a factor contributing to cycle synchrony. The introduction of rats to light–darkness cycles and their removal from the presence of other female rats were stimuli tested for their effectiveness in initiating new cycles, and causing the cycles to be synchronized as a consequence, but, when tested, neither of these environmental changes was found to initiate the observed synchrony of cycles.