A method of measuring narcotic potency is described. Grain weevils (Calandra granaria) are exposed to the narcotic vapour in 2l. round‐bottomed flasks illuminated from the bottom, and narcotic potency measured as number of grain weevils so affected as to be unable to climb the glass walls. Some common anaesthetics have been examined as an illustration of this method. Results are fitted by the probit planeY= A +b1log α+b2logt,whereY= a function of the number of insects affected (probit); α= the thermodynamic concentration of the narcotic vapour;t= time of exposure of insects to the narcotic vapour. The values of the coefficientsb1and b2are discussed in relation to the biological action of the narcotic vapo