ABSTRACTIn Part I, 20Sssqueezed a dynamometer at various levels of rapacity. Heart rate and skin conductance were monitored during each squeeze. In part II, reactivity to the four combinations of the presence and absence of a loud noise and the presence and absence of a strong squeeze was examined. Part HI compared reactivity to the loud noise under the following conditions; as an unfamiliar, surprise stimulus; as a familiar surprise stimulus; as a familiar, expected stimulus. Major conclusions were: (1) Heart rate varies more directly and reliably with motor output than skin conductance; (2) Skin conductance is more sensitive to small cognitive than to small motor effects; (3) Skin conductance is more reactive to stimulus input than to motor output, while the opposite is true for heart rate; (4) A strong familiar stimulus presented by surprise elicits a marked heart rate declarative reaction, usually, but not always, preceded by a smaller accelerative reaction; (5) Baseline changes immediately preceding stimulus onset markedly affect the response to stimulation, and can account for the attenuated reactions observed when a noxious stimulus is preceded by a warning signal.