By use of the Sperber in vivo chicken preparation, infusion of radiolabeled oxalic acid ([14C]oxalate) into the renal portal circulation indicated net excretory transfer of unchanged oxalate. At infusion rates of 0.1 to 100 nmol/min, approximately 26% of the oxalate reaching the kidney was excreted directly into the urine. The excretory transport of oxalate was not altered by infusion of probenecid or terephthalic acid, at rates that blocked completely the excretory transport of simultaneously infused p-aminohippuric acid. Because probenecid and terephthalic acid are also known to inhibit uric acid excretory transport in the chicken kidney, these findings suggest that the transport system for oxalate in the chicken kidney is separate from those handling p-aminohippuric acid and uric acid. The excretory transport of oxalate was decreased by the infusion of alpha-ketoglutaric acid, suggesting that oxalate competes at least in part with other endogenous dicarboxylic acids for uptake at the peritubular membrane.