SUMMARY Electrolytic lesions were produced in the median eminence in a series of rats; the resultant diabetes insipidus varied in severity over the whole range expected from this type of operation, and no part of the median eminence was spared. Plasma corticosterone was estimated both under resting conditions and after ether anaesthesia, and at different intervals after the induction of polyuria. Although the levels under both conditions were variable, the variability was not correlated with the severity of the polyuria, and the mean values were in the normal range.