SUMMARY Nomograms which enable the concentration of halothane or trichloroethylene delivered from a Halox vaporizer to be read directly are presented. The mathematical derivation of the nomogram is shown in the appendix. Similar nomograms can be constructed for any volatile agent whose vapour pressure does not vary greatly over the range of liquid temperature in clinical practice and which is not used in vapour concentrations above about 7 per cent. There are slight inaccuracies in the case of halothane when the concentration is above 5 per cent and at the same time the temperature of the liquid is unusually low or high. These are discussed and are unlikely to be clinically significant. These nomograms might prove useful in practice since the need for mental calculation is avoided.