Renal pathways for excretion of sulfadiazine has been studied by standard clearance technique in the rabbit. 1. Large part of sulfadiazine filtered in the glomeruli is reabsorbed in the tubules, as visualized from the fact that Csd (clearance of sulfadiazine) amounts only a fraction of simultaneously measured Ccr (GFR). 2. Csd changed linearly with the rate of urine flow, whether it is increased by the duir etics or decreased by clamping u reter. 3. Csd remained unchanged until the plasma level of the Csdremained unchanged drug reached 10.0 mg%, and the amount transported in the tubules increased linearly with the increase in the load, exhibiting No maximum capacity for transport. 4. Csd was increased by 2,4-dinitrophenol which is an uncoupling agent of oxidative phosphorylation and decreased by probenecid which is on uricosuric agent. 5. During sodium bicarbonate infusion net secretion of sulfadiazine by tubules observed. All the evidences obtained in the rabbit indicated that sulfadiazine was reabsorbed by active, energy-requiring, or passive, simple diffusion, process, and secreted simultaneously by a probenecid-sensitive, active procss.