The measurement of arterial plasma concentration of hypoxanthine in 16 healthy newborn infants showed a prominent increase after birth compared with the umbilical cord levels. Peak values were found 10-20 min after birth with a maximal value of 11.9 mumole/liter in the normal newborns. In five asphyxiated infants the postnatal pattern of hypoxanthine was the same as for normal infants but the increase was even more prominent. A mean peak value of 33.7 mumole/liter was noted in the asphyxiated infants. Our results indicate that the hypoxanthine concentration is influenced not only by the degree of hypoxia, but also by the peripheral circulation and the time interval between the hypoxic event and the blood sampling. It is concluded that hypoxanthine assay may be of clinical value for the detection od past hypoxia as a semiquantitative test.