Since the method of transuterine insufflation to determine tubal patency or occlusion was devised (November, 1919), I have regarded the postmenstrual phase as the most suitable time in which to apply the test. From an analysis of 1,000 consecutive insufflations, it has become clear that the week following a regular period should practically always be preferred and for the following reasons: ACTIVITY OF UTERINE MUCOSA The uterine mucosa is practically never completely at rest at any phase of the menstrual cycle. Immediately after cessation of the menstrual flow there is already evidence of epithelial proliferation, and the process of regeneration may be said to be superimposed on that of desquamation. But the mucosa is at its thinnest during the first few days following the menses, and therefore offers the least obstruction to the introduction of the uterine cannula. Every day thereafter the superficial portion of the endometrium takes on an