Rats were exposed to an altitude of 14,000 ft for up to 90 days, either in natural cold or at 23° C. An initial tendency to anoestrus was probably due to the sudden change of environment. After about 30 days there was prolonged oestrus, probably as a result of low oxygen pressure. Oestrus was not as prolonged among the cold-exposed animals as among those at 23° C, evidently owing to the anoestrous effect of cold. Recovery usually occurred on descent to sea level.
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