1. Stimulation of left atrial receptors by distending balloons located at the junction between the pulmonary veins and the left atrium and in the body of the left atrium resulted in a diuresis in anaesthetized dogs.2. In the same dogs the diuresis was not consistently associated with a reduction in the antidiuretic activity of plasma as compared with that during the initial control period. The antidiuretic activity of dog plasma was assayed in water-loaded rats anaesthetized with ethanol.3. Intravenous injections of vasopressin suppressed a water diuresis in water-loaded dogs anaesthetized with chloralose. The changes in antidiuretic activity of dog plasma which resulted from these injections were reliably detected by the bio-assay in the rat.4. It is concluded that the diuretic response to the stimulation of left atrial receptors is not accompanied by a reduction in the plasma antidiuretic hormone level.