Clearance of sodium iothalamate was estimated from plasma elimination curves obtained from 90 patients having varying renal function, after a single intravenous injection of 125I-sodium iothalamate. The usefulness of sodium iothalamate as a GFR-indicator with this technique was tested by a strict statistical comparison with conventional inulin clearance as a reference. The regression line, covering the clearance range 2-163 ml min-1, Clinulin = 1.08.Clsodiumiothalamate-3.7, (n = 84, r = 0.85, SEE = 2.3 ml.min-1) was not significantly different from the identity line, which means (i) that extrarenal elimination could not be detected, and (ii) that I125-sodium iothalamate should be regarded as a useful glomerular marker for single intravenous injection studies. An open two-compartment model of mamillary type was found to give an adequate representation of the plasma disappearance curve. The results are not critically dependent on the choice of approximating function. The method by Sapirstein et al., the method by Nosslin and a power-law method were used for comparison and were found to give the same results. However, the present type of curve fitting analyses requires frequent blood sampling or external counting over a long period in order to give reliable estimates of GFR. This circumstance makes these methods less attractive for clinical use.