1. There is a fairly high individual constancy as regards sensitivity to barbiturates; individuals which are narcosised by a median hypnotic dose are likely to show the same response when the test is repeated.
2. There is less individual constancy as regards the duration of narcosis produced by barbiturates. Such constancy can be demonstrated in groups of mice selected as regards sensitivity, but in unselected groups we could not demonstrate any constancy.
The expenses of this research were defrayed by a grant from Messrs. Imperial Chemical Industries (Dyestuffs Group), for which the authors express their thanks.
We also thank Dr. W. O. Kermack for his kind assistance in the mathematical analysis of the results.