The pressures required to abolish the righting reflex of mice have been measured for five gaseous anaesthetics. The ED50 pressures (in atmospheres) found were: N2, 35; CF4, 19; SF6, 6.9; N2O, 1.5; CF2Cl2, 0.4. These data have been collated with other experimental results using mice, in order to obtain a self-consistent set of data for eighteen anaesthetics. With these data the correlation of anaesthetic pressure with olive oil solubility (Meyer-Overton hypothesis) is shown to be superior to that with hydrate dissociation pressure. The best solubility correlation is obtained with solvents of solubility parameter 9 ± 1.