Noise-free and enlarged P loops of the Frank lead system were obtained from 164 normal Japanese males and 137 females (mean age: 34.4 and 33.2 years, respectively) with computer-averaging technique of 8 consecutive beats. Automated measurements concerning 17 parameters of the P loop (P durations, magnitudes, and directions of 3 planar, spatial maximal, and mean polar P vector) were carried out. At the same time, sex and age differences were statistically analyzed. I. Sex differences: Significant sex differences were found in 5 items in the first age group (20-29 years), 3 items in the second (30-39 years), and 7 items in the third (40-49 years). II. Age differences: In males, the effect of aging was observed in 5 parameters out of 17, in time durations extended and maximal vectors displaced posteriorly. In females, changes with advancing age were observed in 13 items out of 17, thus leaving only 4-parameter, including frontal maximal P vectors and directions of mean polar P vectors as unaffected.