Changes in umbilical cord blood pH, PO2 and PCO2 were measured when blood was stored in preheparinized polyethylene syringes in the refrigerator for up to 6 h after delivery, and when there was delayed sampling up to 1 h after delivery from umbilical cord segments left at room temperature. Blood stored in the refrigerator usually showed a decrease in pH and increases in PO2 and PCO2, but the mean rates of change were small: -0.005 units/h for pH; +0.06 kPa/h for PCO2 and +0.03 kPa/h for PO2. Changes in the three variables in blood collected from umbilical cords up to 1 h after delivery were small and not systematic. Much of the variation was within the limits of accuracy of the ABL-3 (Radiometer) blood-gas analyser. The results showed that while immediate collection and analysis of umbilical cord blood is advisable for the greatest accuracy, it is not essential. As long as the delay is not excessive, the results can still be used as a useful guide to the biochemical condition of an infant at birth.