Renal clearances of diatrizoate sodium I 125 or iothalamate sodium I 125 measure 90% to 100% of simultaneous classic clearances of inulin; clearances of iodohippurate sodium I 131 or iodopyracet I 131 are 84% and 79%, respectively, of simultaneous clearances of aminohippurate sodium. Plasma binding (8% to 27% of diatrizoate sodium I 125 or iothalamate sodium I 125; 64% to 70% of iodohippurate sodium I 131; 29% to 42% of iodopyracet I 131; 7% to 25% of aminohippurate) is revealed by ultrafiltration studies. Binding is labile and dissociable. Correcting clearances of diatrizoate sodium I 125 or iothalamate sodium I 125 for plasma binding yields values exceeding those of insulin clearances. Uncorrected, they are satisfactory substitutes for inulin clearance in man. Clearances of iodohippurate sodium I 131 or iodopyracet I 131 approximate adequately clearances of aminohippurate when carrier iodopyracet is added and the clearances are corrected for plasma binding to provide an estimate of total clearance.