Infusion of para-aminohippurate (PAH) at rates below the maximum rate of transport (Tm) of PAH in the rat resulted in a significant decrease in the fractional excretion of oxalate (FEox) from 128.1% to 113.9% (P less than 0.01). Fractional delivery of oxalate (FDox) to the early proximal tubule, however, was unchanged from control values, whereas FDox to the late proximal tubule was significantly decreased from 126.4% to 107.4% (P less than 0.01). Infusion of PAH at rates above Tm of PAH resulted in a decrease in FDox to the early proximal tubule to 105.3% and to the late proximal tubule to 105.5%, and in FEox to 100.5%. These changes were not the result of alterations in urinary sodium or bicarbonate excretion. Microinjection studies indicated that PAH did not affect the tubular absorption of oxalate. These studies suggest that PAH inhibits the tubular secretion of oxalate and that there may be more than one secretory system for oxalate with differing affinities for oxalate and PAH in the early and late proximal tubules.