The effects of uremic sera from patients with different kidney disorders on PAH uptake by isolated renal tubules of the rabbit were investigated. These sera were found to depress PAH uptake when compared to normal control sera. The inhibitory effect of the uremic sera was markedly reduced by hemodialysis or peritoneal dialysis of the patient or in vitro dialysis of the sera. The inhibiting effect was also noted in peritoneal dialysate. The in vitro inhibition of PAH transport demonstrated in this study indicates a mechanism that may explain poor extraction of PAH in patients with renal insufficiency and makes difficult the interpretation of certain renal function studies on them.