1. The intraatrial balloon technique was used to study the effect of distension of the left atrium on the urinary flow of anesthetized dogs. The balloon was inflated with 0.3, 0.4 or 0.5 ml saline/kg body weight, respectively, in three experimental groups, each containing 10 mongrel dogs. Intraatrial pressure, arterial blood pressure, ECG and respiratory excursions were continuously monitored in all experiments. Urine flow, inulin and p-aminohippuric acid clearance, urine osmolarity, total sodium and total potassium excretion were measured during the control period, after inflation of the balloon and 30 min after deflation. 2. Left atrial distension with 0.3 ml/kg caused essentially no increase in diuresis. Inflation with 0.4 ml/kg induced a 60% increase of mean urinary flow with respect to the control period. Inflation with 0.5 ml/kg elicited a mean increase in diuresis of 110%. 3. When it occurred, increased diuresis was accompanied by a decrease of urine osmolarity that was compatible with a decrease of antidiuretic hormone activity, and by an increase in total sodium excretion. There were no statistically significant changes in glomerular filtration rate and effective renal plasma flow. 4. The demonstration of a "threshold" for the distension of the left atrium that is capable of inducing diuresis in the anesthetized dog and of overcoming strong opposing hemodynamic effects provides additional indirect evidence for the role of the left atrial stretch receptors in the regulation of body fluid volume.