Seasonal variations in the response of rats to the antidiuretic hormone.

The antidiuretic response of unanaesthetized rats to intravenous doses of vasopressin has been compared in the different seasons of the year. Significant seasonal differences in sensitivity to the hormone were found; thus the rats responded to smaller doses in spring and summer. Discrimination between doses appeared to be best in summer. Relative variability in response seemed to be least in winter. There appeared from season to season to be a concomitant variation in the same or opposite direction between sensitivity in terms of the average minimum effective dose and environmental factors such as temperature, barometric pressure and relative humidity.