65 Naval helicopter pilots were examined to determine the stability and correlates of spontaneous autonomic activity, the effect of stimulation on spontaneous responses and the possible use of spontaneous activity as a measure of general autonomic lability. Over a 48-hr. period spontaneous GSR was found to be moderately stable (rho = .69) but spontaneous heart rate (HR) was less reliable (rho = .36). Spontaneous HR and GSR were independent of each other and neither was consistently related to EEG measures, heart rate, basal conductance, skin temperature, respiration, or blood pressure. Spontaneous activity during a resting period was related to that found during stimulation. For spontaneous GSR there was an initial increase in activity to each stimulus but rapid adaptation occurred to repetitive presentations. Spontaneous HR showed no change during stimulation. Ss with a large number of spontaneous responses, especially GSR, showed more reactivity to initial stimuli and less adaptation to repetitive stimuli than Ss with few spontaneous responses. (PsycINFO Database Record (c) 2006 APA, all rights reserved)