“…These assumptions are indirectly favoured by the finding that, whereas there is no marked sex variation in the hydrolysis rate of acetylcholine in human plasma (Levine and Hoyt, 1949;Callaway, Davies, and Rutland, 1951), a statistically significant difference is found in the hydrolysis rate of procaine in normal male and female plasmas (Davis et al, 1953). Similarly, whereas the hydrolysis rates of acetylcholine in horse serum (Glick, 1941 ;Ginzel, Klupp, and Werner, 1951) and of succinylcholine in dog plasma (Hall, Lehmann, and Silk, 1953) were as fast or faster than that in human plasma (Evans et al, 1952;Tsuji and Foldes, 1953), the hydrolysis rate of procaine in the plasma of these species, and of other mammals, is many times slower than in human plasma (Aven, Light, and Foldes, 1953). To clarify these problems it will be necessary to study the hydrolysis rate of various substrates (acetylcholine, SDC, procaine, etc.)…”