In a recent study1it was found that the various component phenomena of estrus in the rat may be classified in two categories: (1) the specific proliferative effects of estrogen and (2) those effects which are secondary to hyperemia. The latter effects can be reproduced by nonspecific hyperemic agents such as yohimbine. However, when they result from the action of estrogen they are due to the acetylcholineliberating properties of this hormone, as demonstrated by Reynolds.2 In view of the large proportion of the estrous phenomenon in animals which is dependent on hyperemia, it seemed likely that the role of hyperemia was probably even more important in the periodic menstrual bleeding of the human female. Considering the mechanism of this hyperemia, it would presumably also be under the influence of at least the parasympathetic nervous system. This relationship of the nervous system to uterine hyperemia might readily account for