The structural specificity of the responses to oxytocin by mammary myo-epithelial cells was tested in women and rabbits 1) by determining the threshold dose of oxytocin necessary to produce a detectable increase in intramammary pressure; 2) by comparing the milkejecting potency of synthetic chemical analogues of oxytocin; and 3) by comparing the milk-ejecting potency of acetylcholine. The results obtained indicate that the human myo-epithelial cell is more sensitive than the rabbit‘s to the action of oxytocin, that small structural changes in the oxytocin molecule alter its milk-ejecting activity more in women than in rabbits, and that the mammary myo-epithelial cell in rabbits is much more sensitive to the action of acetylcholine than in women. We conclude that in women there is a greater structural specificity of the response of the mammary myo-epithelium to oxytocin, a hormone natural to both species. (Endocrinology81: 515, 1967)