The disposition of inulin was studied in 30 healthy male and 10 healthy female volunteers, and 10 patients with stable chronic renal failure (mean creatinine clearance 45 ml.min-1) following intravenous infusion of 70 mg.kg-1 over 5 min. Plasma concentrations fell rapidly initially but the rate of decline decreased continuously over 8 h and a linear terminal elimination phase could not be identified. Inulin was excreted rapidly by the subjects with normal renal function and 97.3% of the dose was recovered in the urine in 8 h. There was a progressive highly significant fall in the renal clearance of inulin after 2 h as plasma concentrations fell below about 150 mg.l-1. Six to 8 h after administration the clearance was less than 50% of the initial value in the healthy volunteers and the corresponding fall in the renal patients was 33%. The concentration-dependent renal clearance of inulin was confirmed in "step-up" and "step-down" constant infusion studies in which clearances were measured at mean plasma concentrations ranging from 35.2 to 186.7 mg.l-1. These studies virtually excluded time, changes in posture and urine flow rate as important factors. There was no statistically significant fall in clearance during the first 2 h and kinetic analysis was based on data obtained over this time.(ABSTRACT TRUNCATED AT 250 WORDS)