The object of this study was to establish the nature of the intra-mammary pressure changes that occur in the udder of the lactating dairy cow during different phases of lactation. The author was, in addition, interested in the study of the various factors responsible for these changes. Results have shown marked and rapid fluctuations in the intra-mammary pressure at the time of milking. Stimulation of milk ejection by manipulation of the udder caused a spectacular increase in the initial intra-mammary pressure. On milking a drop in the pressure was recorded. The extent of this fall in pressure was found to be influenced by the amount of milk removed from the udder. On complete milking the intra-mammary pressure dropped to the minimum level. The pressure in the hind quarters was found to be in most cases higher than in the fore-quarters indicating that the level of the intra-mammary pressure in each quarter was under some influence of the quantity of milk present. A fall in the intra-mammary pressure of a milked quarter had no effect on that of the other unmilked quarters. Intra-mammary pressure subsequent to the injection of oxytocin before milking as well as after complete milking was recorded. The administration of oxytocin under both of these conditions resulted in a rise of the pressure, which, however, was not well marked. Considerable effect of the conditioned reflexes and of the emotional state of the animal on the intra-mammary pressure was observed during this work. / Land and Food Systems, Faculty of / Graduate