Serum, taken from rats under ether anesthesia after injection of hypertonic saline, contained antidiuretic activity in a concentration equivalent to 1 to 3 millipressor units of pitressin per ml.

The active material was retained by a collodion ultrafilter and, on electrophoretic analysis, moved with the beta globulins. Dialysis against 0.2 N acetic acid appeared to dissociate the complex.

The antidiuretic factor was found in serum after stimuli known to cause release of pituitary antidiuretic hormone. Inactivation by thioglycolic acid and by proteolytic enzymes paralleled the inactivation of standard pitressin. Control experiments made it unlikely that serotonin, or any other substance released in the clotting of blood, could have contributed to the antidiuretic action. For these reasons it was concluded that the factor studied was native pituitary antidiuretic hormone circulating as a definite protein-peptide complex.

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