Heparinizied human and dog blood were equilibrated with low concentrations (0.7 to 1.4 per cent) of anesthetics and assayed for anesthetics by gas chromatography. Partition coefficients for human blood (Hb 15 g per cent) at 37.5 C were: 0.149 for ethylene; 0.574 for cyclopropane; 2.50 for halothane; 9.96 for diethyl ether. Partition coefficients for dog blood were: 0.182 for ethylene; 0.765 for cyclopropane; 3.11 for halothane; 10.07 for diethyl ether. Ethylene and cyclopropane showed increasing solubility, whereas halothane and diethyl ether showed decreasing solubility, with increasing hematocrit or hemoglobin concentration. Comparison with previously reported values suggests that ethylene and cyclopropane have higher partition coefficients and diethyl ether a lower partition coefficient when low concentrations of anesthetic are used for the determination. Gas chromatographic head space analysis after equilibration of 3-ml blood samples in small sealed glass tubes provides an accurate method for the assay of several inhalation anesthetics in a single blood sample. The method is also convenient because it allows storage of the samples for several hours before assay.