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Effects of esterification. Allen and Doisy and their co-workers [1924] in their early work on ovarian extracts found that a given amount of active substance was much more efficient when administered as a series of small doses than when given in one injection. With the advent of more purified preparations and of aqueous solutions, divided dosage became even more advantageous. Thus Laqueur & de Jongh [1929] found that more than 30 times the total amount of extract was required to give the same effect when the whole dose was given in one injection instead of in six separate injections over 36 hours. These results were confirmed by Dodds and his co-workers [Allan et al. 1928] and by Marrian & Parkes [1929], and could obviously-be explained by supposing that a single dose, especially in aqueous solution, was quickly absorbed and excreted or destroyed, and that the duration of threshold concentration in the blood necessary for a positive response could only be achieved either by a massive single dose or by a series of injections. Soon afterwards it was discovered by Butenandt [1930] that oestrone benzoate had a prolonged action, a finding afterwards confirmed by David et al. [1935] for oestradiol monobenzoate. In the meantime Zondek [1934] had found, as might be expected from Butenandt's results, that the beiizoate did not disappear from the body so rapidly after injection as the free hormone. Subdivision of the total dose is thus not necessary for achieving the maximum effectiveness of the benzoate as it is in the case of the free hormone. It follows from this that extraordinary differences in the apparent relative activities are obtained according to the method of test used for comparing the free hormone and the benzoate. Other esters of the oestrone series which have been examined include oestriol triacetate [Marrian, 1930], which in the ordinary routine test was about two-thirds as active as oestriol, and oestradiol diacetate and dibenzoate which were found by David et al. [1935] to show much less intense, but more prolonged actions than the free hormone, the minimum effective dose of the dibenzoate being 170 times greater than that of the free hormone. This work on the effect ofmode of administration on the efficiency of oestrone and oestradiol has been paralleled remarkably closely by recent tests on the male hormones. Both androsterone and testosterone are rapidly excreted or destroyed after subcutaneous injection. An extraordinary increase in effectiveness of a given dose can be induced if absorption is delayed by subdividing the daily dose, increasing the volume of oil in which the hormone is dissolved, adding fatty'acid to the solution or esterifying the hormone [see Miescher et al. 1936, and Parkes, 1936].

Relative duration of action of various esters of oestrone, oestradiol and oestriol