1. The purpose of this study was to determine whether the baroreceptor reflex system affects the secretion of atrial natriuretic factor directly or indirectly during acute volume expansion.

2. Lactated Ringer's solution containing low mol. wt. dextran was infused at 20 ml/kg for 1 h into dogs in which baroreceptors had been denervated surgically (Vx), dogs in which the autonomic system had been blocked with hexamethonium (Hx) and control dogs.

3. The plasma noradrenaline level was significantly higher in the Vx group and lower in the Hx group than in the control group throughout the experiment. The plasma levels of arginine vasopressin in the Vx and Hx groups were significantly higher than in the control group.

4. The plasma atrial natriuretic factor levels in the three groups showed similar increases during and after volume expansion.

5. The plasma atrial natriuretic factor level was significantly correlated with the right atrial pressure during volume expansion.

6. From these results, it seems unlikely that changes in the plasma level of atrial natriuretic factor during volume expansion are regulated by the baroreceptor reflex directly or indirectly by systemic changes in the sympathetic nervous system or arginine vasopressin secretion.

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