Proximal and distal tubular segments of the subcapsular nephron and segments of the ascending limb of Henle’s loop were dissected freehandedly from lyophilized kidney section of moderately dehydrated rats after furosémide (100 mg/kg body weight). An 80% inhibition of Na+K+ ATPase activity after furosémide was obtained by quantitative histochemical measurement in the ascending limb of Henle’s loop and in the distal convoluted portion only. These results demonstrate: First, that furosémide acts mainly within the ascending limb of Henle’s loop and the distal convoluted portion where the Na+K+ ATPase likely mediates sodium transport. Second, that a primary influence of the diuretic on the metabolism involved in cation transport i