&NA;Infusion of &agr;‐human‐atrial natriuretic factor (&agr;‐h‐ANF) into pentobarbitol anesthetized dogs (n= 10) at 0.0025, 0.005, 0.01, and 0.3 μg/kg/min was performed to differentiate the physiologic actions of atrial natriuretic factor from its pharmacologic actions. The lowest doses of atrial natriuretic factor infusion resulted in circulating levels that were previously produced by 0‐10% saline volume expansion. At the lowest infusion rate, circulating ANF increased 31 ± 3 pg/ml, resulting in a significant increase in absolute sodium excretion, fractional excretion of sodium, and fractional excretion of lithium, and a significant decrease in urine osmolality. A greater change in circulating atrial natriuretic factor (96 ± 12 pg/ml) was required to significantly decrease right atrial pressure, cardiac output, and plasma renin activity, and to increase systemic vascular resistance and total and fractional excretion of potassium. The highest dose of atrial natriuretic factor infused was required to decrease arterial pressure and renal vascular resistance. The present study demonstrates that 1) atrial natriuretic factor is natriuretic and diuretic at physiologic concentrations; 2) at low concentrations, atrial natriuretic factor appears to decrease the whole kidney proximal tubular reabsorption of sodium and does not affect glomerular filtration rate; 3) a greater (but physiologic) change in circulating atrial natriuretic factor is required to significantly decrease cardiac output, cardiac filling pressure, and plasma renin activity than is required to significantly increase sodium excretion; and 4) a decrease in systemic arterial pressure and vascular resistance does not occur at physiologic concentrations of atrial natriuretic factor. (Circulation Research1987;60:63‐69)