Determinations of umbilical blood pH and PCO2 can be useful for correlating intrapartum fetal heart rate patterns, intrapartum samples of scalp blood, and Apgar scores, for the retrospective evaluation of the management of a particular labor. Occasionally, such data can indicate and clarify needed resuscitative measures and intensive neonatal observation. Cumbersome techniques have been described for the handling of specimens of cord blood to ensure an accurate determination. This study was performed to determine the effect on cord blood pH and Pco2 of room temperature and time from delivery. One hundred five cord venous or arterial determinations had a linear pH decrease versus time at room temperature. The regression slope was -3.66 x 10-4 per minute with a standard deviation of 1.37 x 10-2. The PCO2 showed only a slight increase with time at room temperature, although the scatter was high, with a standard deviation of 3.8 torr. The conclusion is that samples of cord blood drawn for determination of pH and Pco2 can be kept at room temperature in plastic syringes for up to 30 minutes without significant alteration in these values, i.e., pH decrease no greater than 0.04 unit (p < 0.05). © 1984, Mosby. All rights reserved.