The current study evaluates the validity of the PEP computed from a fixed value for the Q-wave onset to R-wave peak (QR) interval and an R-wave peak to B-point (RB) interval that is estimated from the R-peak to dZ/dt-min peak (ISTI) interval. Ninety-one subjects participated in a 90 minute laboratory experiment in which a variety of often employed physical and mental stressors were presented and 31 further subjects participated in a structured 2 hour ambulatory recording in which they partook in natural activities that induced large variation in posture and physical activity. PEP, QR interval, and ISTI were scored and rigorously checked by interactive inspection. Across the very diverse laboratory and ambulatory conditions the QR interval could be approximated by a fixed interval of 40 ms but 95% confidence intervals were large (25.5 to 54.5 ms). Multilevel analysis showed that 79% to 81% of the within and between-subject variation in the RB interval could be predicted by the ISTI with a simple linear regression equation. However, the optimal intercept and slope values in this equation varied significantly across subjects and study setting. Bland Altman plots revealed a large discrepancy between the estimated PEP using the R-wave peak and dZ/dt-min peak and the actual PEP based on the Q-wave onset and B-point. We conclude that the PEP estimated from a fixed QR interval and the ISTI could be a useful addition to the psychophysiologist's toolbox, but that it cannot replace the actual PEP to index cardiac sympathetic control.