[125I]iothalamic acid clearance is used for the estimation of glomerular filtration rate. Therefore, we developed a coupled-column liquid chromatography for the simultaneous determination of iothalamic acid and creatinine in human serum and urine samples to calculate their clearance. The liquid chromatograph consisted of an automatic injector, four columns and four switching valves. Timing of sample injection and switching of valves were controlled by a combination of timer units. Diluted samples were injected into a column of weakly acidic ion exchanger (Asahipak ES-502C, 30 degrees C, C1), and iothalamic acid excluded from the column was purified by gel chromatography on a hydrophilic gel (Asahipak GS-320H, 50 degrees C, C2) and then by ion exchange chromatography on a weakly basic anion exchanger (Asahipak ES-502N, 50 degrees C, C3). Creatinine that was eluted from C1 after iothalamic acid was transferred to Asahipak GS-320H (37 degrees C, C4) and C1 was backflushed until 10 min before the next sample was injected. Samples were injected every 55 min. The mobile phase was a sodium propionate buffer prepared by dissolving 0.035 mol of NaOH and 0.35 mol of propionic acid in 1 kg of ultra pure water, and its flow rate was 1.1 ml per min. Creatinine clearance calculated from the data obtained by this method was higher than that obtained by Jaffé's method, because serum creatinine concentrations obtained by this method were lower than those obtained by Jaffé's method. Iothalamic acid clearance (y) calculated from the data on serum and urine samples of clearance time 105-135 min showed a close correlation with inulin clearance (x) calculated from the data obtained by the anthrone method, for which aliquots of the same samples were used (y = 1.066 x -1.429 ml/min, r = 0.997, n = 15).