Premature ventricular complexes occurred with increasing frequency and in a greater proportion of men according to the work load imposed among 196 middle-aged "high risk" men free of manifest heart disease. The proportion of men with any ventricular ectopic activity increased from less than 3 percent at standing rest to 30 percent at the top work load, and from less than 3 percent at heart rates of 73 to 102 beats/min to over 50 percent at heart rates above 173 beats/min. Between the stress of a submaximal test (to heart rate 150) and a test to maximal oxygen consumption, there was a sizable increase (35 percent) in the yield of premature ventricular complexes and of complex forms and runs. The prognostic importance of ectopic beats during progressive stress tests required systematic study in populations. After an 18 month program of progressive conditioning exercise in these previously sedentary men, several analyses among subjects who adhered well to the program revealed that the work threshold for ectopic activity may have been increased, the proportion of men with ectopic beats diminished, and the frequency of premature ventricular complexes per man diminished. The results are suggestive but not conclusive that exercise favorably affects this aspect of cardiac activity. A more definitive study is indicated, using hygienic efforts to modify multiple influences on ectopic ventricular activity.