Clearance and micropuncture experiments were performed in rats to study the renal handling of oxalate. The 14C-oxalate to 3H-inulin clearance ratio (Cox/Cin) was 1.36 +/- 0.04 and was lowered by probenecid (200 mg/kg) to 1.11 +/- 0.03 (+/- S.E., n = 6, P less than 0.005). An attempt was made to localize the assumed secretion of oxalate in three different micropuncture protocols. In free flow micropuncture experiments single nephron clearance of oxalate was not different when obtained from proximal or distal tubular puncture sites. The fractional delivery of oxalate averaged 0.84 +/- 0.03 regardless of the puncture site from mid-proximal to late distal. This finding indicates a net outflux of oxalate in an early proximal loop since oxalate is freely ultrafilterable. In microperfusion experiments the mean recovery of oxalate ranged from 79--90%. The outflux of oxalate correlated linearly with the tubular load (r = 0.95). The results suggest that no net secretion occurs in superficial nephron segments accessible for micropuncture. Since whole kidney clearances of oxalate always exceeded glomerular filtration rate, it is concluded that net addition of oxalate into the tubular fluid can occur at sites beyond the superficial late distal tubules or is due to higher delivery of oxalate by deep cortical nephrons.