The mechanism of urinary excretion of sulfadiazine was investigated in the dogs by means of stop-flow technique Results of experiments were summarized as follows; 1. The ratios of U/P sulfadiazine to U/P creatinine $(U/P_{SD}:U/P_{cr})$ were always lower than 1 in all nephrons, showed a minimum in the proximal area. 2. $U/P_{SD}:U/P_{cr}$ were not affected by Probenecid or 2.4-DNP, whereas increased significantly by administration of sodium bicarbonate. 3. Probenecid did not alter the stop-flow patterns in alkalotic dog too. 4. $C_{SD}$ (clearance of sulfadiazine ) was appreciably influenced by change in urinary PH, or flow rate. All evidences lead to the conclusion that sulfadiazine reabsorption is passively transported by proximal tubules. No clue for active process, either secretory or reabsorptive, was obtained.