Body surface potential maps were obtained in 50 normal adults using the heart potential map system designed by Yamada. Group A (younger adult group) consisted of 30 persons whose ages ranged from 20 to 39 years. Group B (middle-aged group) included 20 persons whose ages ranged from 40 to 59 years. Although there were slight variations in the pattern of isopotential distribution among the subjects studied, a consistent pattern with a similar sequence was observed in both study groups. However, there were some parameters which significantly differed between groups A and B, including the earliest appearance time of the anterior negative potential (p < 0.05), the anterior potential minimum (p < 0.001), the posterior positive potential (p < 0.001), the midsternal minimum (p < 0.001), the largest potential minimum (p < 0.05), the largest potential maximum (p < 0.005), the "reversal" pattern (p < 0.001), the multipolar potential distribution in the upstroke (p < 0.001) and the amplitude of potential minimum in the periods 50 msec (p < 0.005) and 60 msec (p < 0.01) of ventricular depolarization. Accordingly, the data obtained regarding the time relationship between the bipolar and multipolar phases, the distribution and time course of the potential as well as the dynamic changes in potential voltage, particularly the age-related potential pattern will serve as a basis for further study of BSPM.