1. Stimulation of atrial receptors by distension of a balloon in the left atrium of anaesthetized dogs results in a reflex diuresis mediated by a humoral agent of unknown identity.

2. In previous investigations, a substance was recovered in a low molecular weight fraction from Bio-Gel P-2 (100-1800 daltons) and shown to be related to the reflex diuresis.

3. In this investigation, the active Bio-Gel P-2 fraction was partitioned with ethyl acetate at pH 11.0, with subsequent partition of the aqueous phase with ethyl acetate at pH 7.4; the activity of the humoral agent was soluble in ethyl acetate at pH 7.4.

4. These investigations suggest that the humoral agent is a low molecular weight, lipophilic and weakly acidic substance.

5. The substance is not antidiuretic hormone (ADH) as vasopressin is insoluble in ethyl acetate at pH 7.4.

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