Analysis of the postexercise electrocardiographic responses to the third annual test of maximal exercise with a multistage treadmill test (rather than a step test) in 191 middle-aged American men revealed inter-observer differences in interpretation in 5% of the men. Sequential variations which were found in 11% of 186 men who were tested three times in a period of three years demonstrated the potentiality for changes in this response. Only men with changing electrocardiographic responses showed significant intergroup differences in resting blood pressure and serum lipid concentrations. Covariance analysis revealed significant differences in age-adjusted, and particularly in height-adjusted mean values for resting blood pressure in relation to electrocardiographic responses to maximal exertion. There were no comparable differences in age-adjusted cholesterol levels.