1. The effect of partially purified rat atrial natriuretic factor on sodium efflux from sodium-loaded human erythrocytes was studied.
2. High molecular weight (10000–30000) and low molecular weight (3000–10000) fractions of atrial extract were prepared by gel filtration; they had natriuretic activity in rats.
3. Neither fraction affected sodium efflux in erythrocytes.
4. The results suggest that atrial natriuretic factor acts in the kidney by mechanisms other than through inhibition of the Na+-K+ exchange pump or the Na+-K+ cotransport system.