Simultaneous clearances of inulin, urea and creatinine were compared in 41 babies of 26-40 weeks gestation on 122 occasions during the first month of life. In each case creatinine was measured by a reaction rate method, and in thirty specimens it was also measured after adsorption onto resin. Urea clearance averaged only 62% of inulin clearance (P less than 0.001), and was a poor marker of glomerular filtration. Creatinine clearance measured by resin adsorption equalled inulin clearance, but the assay is manual and not suitable for routine clinical use. Creatinine clearance measured by reaction rate analysis underestimated inulin clearance by a quarter (P less than 0.01) because this automated method overestimated plasma creatinine by an average of 22 mumol/l. Urinary creatinine excretion was 72 +/- 17 nmol/kg per min (mean +/- S.D.) during the first week of life, and 66 +/- 13 nmol/kg per min in weeks two to four, and was not influenced by gestation or body size. Using these values, glomerular filtration rate, urine flow, and the urinary excretion rates of substances may be estimated from measurements made on plasma and untimed urines. Although these estimates are imprecise, with 9% confidence limits of 62-161%, they are useful in clinical practice because they avoid the need to make accurately timed collections of urine.