Rates of creatinine clearance were measured in normal newborn babies by collecting a timed specimen of voided urine and estimating plasma creatinine. Each of the 17 babies had the estimation on day 1 and on day 6 of life. The mean rate on day 1 was 17·8 ml/minute per 1·73 m2, and there was a significant and systematic increase by day 6 when the mean rate was 36·4 ml/minute per 1·73 m2. The findings are important when assessing fluid requirements and drug dosage in the newborn.