Serum creatinine, uric acid, and hypoxanthine and xanthine concentrations were determined in 17 mother-infant pairs at the time of delivery. Creatinine and uric acid levels were nearly similar, but hypoxanthine and xanthine were more than twice as high in the blood of the infants than in the blood of their mothers. In the same newborns the urinary excretion of creatinine, uric acid, hypoxanthine and xanthine, uracil, cAMP, and cGMP was measured on the first and fourth day of life. Creatinine, uracil, and cAMP increased, hypoxanthine and xanthine, and cGMP decreased, whereas the output of uric acid was nearly the same on both days. Correlations of the excreted substances to each other were calculated.