The gas chromatographic determination of carboxyhemoglobin in blood has been improved by the use of a head space technique to measure the carbon monoxide (CO) released from blood. To improve sensitivity, the CO is reduce to methane before detection by the flame ionization detector (FID). The time for a single determination is reduced to five minutes by backflushing the column as soon as the methane has been eluted. This automated procedure is suitable for routine analysis of large batches of samples. Results are compared with those obtained using a CO-Oximeter.