Measurement of umbilical cord blood acid-base status is routinely carried out in many obstetric centers. Umbilical cord blood pH, PO2, PCO2 and SBE (standard base excess) may change between clamping of the cord and analysis due to diffusion and metabolism. It was the aim of this study to evaluate these changes separately in artery and vein blood during storage in the clamped umbilical cord. The umbilical cords from 11 normal term deliveries were clamped immediately after delivery and kept at room temperature. Samples of artery and vein blood were drawned separately 1, 5, 10, 15 and 30 minutes post partum and pH, PO2 and PCO2 were measured and SBE calculated. As demonstrated in table I, the vein blood pH and PO2 values were higher and PCO2 values lower than in the artery blood. The changes over time are given in figure 1 with the 1 min values as points of reference. Changes within the first 15 minutes were modest. A significant fall in vein pH, vein SBE and artery PO2 and a significant rise in artery PCO2 was found. Greater variation in the changes over time was found in artery than in vein blood, being most evident for the 30 min values. Although significant changes in the umbilical cord acid-base values and PO2 do occur when the blood is stored in the clamped cord, we find these changes to be minor and we conclude that sampling can be postponed for up to 15 minutes after delivery.