Renal transport of¹⁴C-oxalate was studied in the rat by clearance and micropuncture techniques. The ultrafilterability of oxalate was 0.98±0.02 (n=7). Fractional clearance of oxalate was significantly above unity in antidiuresis and volume expansion: mean 1.24 ±0.04 (n=115). Pyrazinamide (1.1·10⁻³ mol/kg BW) and probenecid (0.35·10⁻³ mol/kg BW) had no significant effect on oxalate clearance. P-aminohippurate (1.45·10⁻³ mol/kg BW) and urate (0.48 ·10⁻³ mol/kg BW) depressed the fractional clearance of oxalate significantly from 116 to 91 and from 125 to 90%, respectively. Excess excretion of¹⁴C-oxalate over³H-inulin was invariably demonstrable in peritubular microperfusion experiments (n=5) and in microinfusions underneath the kidney capsule (n=4). Together with the first 50% of³H-inulin 58±2% of the total¹⁴C-oxalate were excreted in the peritubular microperfusions, and 64±3% in the subcapsular microinfusions. In tubular microinfusion experiments (n=36) urinary¹⁴C-oxalate recovery was almost complete after early proximal microinfusion (93±4%) and complete after late proximal microinfusion (102±4%). In continuous microperfusion experiments of proximal tubules (n=42) a small but highly significant outflux of¹⁴C-oxalate of 7% per mm perfusion distance was found. The data suggest that oxalate is freely filterable at the glomerular site. A small but significant amount of oxalate is reabsorbed in the proximal nephron. Most likely at the same site and in the pars recta oxalate is secreted and tubular load increased to 124% of filtered load. This amount is excreted in final urine. The secretion of oxalate is inhibited by organic acids which are known to be secreted by the proximal tubule and the pars recta.