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Blood pressure determinations were made over a period of two and a half years in specially trained dogs. Readings were taken under varying conditions, such as presence of strangers in the laboratory, changes in experimenter, noise and presence of strangers in the kennels, changes in established routine, and exposure to a bitch in heat. The responses varied greatly according to two factors, the natural emotional reactivity of the animal, and the degree of training. Highly trained and conditioned animals showed much greater variations in blood pressure in response to minor changes in the environment. Possibly the vasomotor system of these dogs had become irritable, for the change in blood pressure was grossly disproportionate to the trivial emotional stimulus. A similar sensitivity perhaps results in man from modern living. It is also possible that repeated emotional cardiovascular responses produce secondary changes in the kidneys and cardiovascular system, causing essential hypertension. 'These studies show that many common laboratory procedures, usually considered innocuous, may engender marked emotional disturbances which are not grossly evident in trained dogs.'