In-vivo and in-vitro partition coefficients for halothane were determined in the rabbit for blood, brain, heart, kidney, liver and skeletal muscle. Statistical analysis showed that for blood, kidney, liver and muscle the differences between the in-vivo and in-vitro coefficients were less than about 10%, most probably less than 5%, and in any case not statistically significant. For brain and heart the differences were statistically significant but may be attributable, not to any fundamental difference, but to systematic experimental errors arising mainly from the difficulty of obtaining homogeneous samples sufficiently large for submission to both techniques.