SUMMARY Following a single intravaginal injection of oestrogen, the number of mitoses and thickness of the vaginal epithelium of the mouse are increased. An assay utilizing such responses is of very short duration and is ten times as sensitive and four times as precise as the standard intravaginal assay under optimum conditions. Three natural and three synthetic oestrogens are of approximately equal potency when assayed by this method. Testosterone, progesterone, cortisol, cortisone and vitamin A were found to be without action on the response to oestrogens, except that the first three produced mitotic activity at very high doses.