Ambulatory recording of the preejection period (PEP) can be used to measure changes in cardiac sympathetic nervous system (SNS) activity under naturalistic conditions. Here, we test the ECG T-wave amplitude (TWA) as an alternative measure, using 24-h ambulatory monitoring of PEP and TWA in a sample of 564 healthy adults. The TWA showed a decrease in response to mental stress and a monotonic decrease from nighttime sleep to daytime sitting and more physically active behaviors. Within-participant changes in TWA were correlated with changes in the PEP across the standardized stressors (r = .42) and the unstandardized naturalistic conditions (mean r = .35). Partialling out changes in heart rate and vagal effects attenuated these correlations, but they remained significant. Ambulatory TWA cannot replace PEP, but simultaneous recording of TWA and PEP provides a more comprehensive picture of changes in cardiac SNS activity in real-life settings.