The present investigation was undertaken to study the kinetics of 99mTc-labelled diethylenetriaminepenta-acetate (DTPA) as compared with inulin and 51Cr-labelled ethylenediaminetetra-acetate (EDTA). Twenty patients with various degrees of decreased renal function were studied. The renal clearance, plasma clearance and volume of distribution of all three tracers were measured after a simultaneous single injection. The average renal clearance ratio 99mTc-DTPA to inulin was 0.97; the average renal clearance ratio 99mTc-DTPA to 51Cr-EDTA was 1.02. In all patients the plasma clearance of inulin exceeded that of 99mTc-DTPA. No difference was seen between the plasma clearance of 99mTc-DTPA and 51Cr-EDTA. The plasma clearance of all three tracers overestimated the simultaneously measured renal clearance; on average this was, for 99mTc-DTPA 5.7 ml/min, for 51Cr-EDTA 6.0 ml/min and for inulin 8.1 ml/min. The plasma clearance of 99mTc-DTPA correlated well with the renal clearance of inulin, but overestimated this by 3.5 ml/min on average. The volume of distribution of inulin was less than that of 99mTc-DTPA and 51Cr-EDTA. No difference was seen between the volume of distribution of 99mTc-DTPA and 51Cr-EDTA. It is concluded that the difference in the kinetics of 99mTc-DTPA and 51Cr-EDTA in patients with decreased renal function was small and without clinical relevance.(ABSTRACT TRUNCATED AT 250 WORDS)