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Description: Research on effort and motivation commonly measures how the sympathetic branch of the autonomic nervous system affects the cardiovascular system. The cardiac pre-ejection period (PEP), assessed via impedance cardiography, is a widely-used sympathetic outcome, but assessing PEP requires identifying subtle points on cardiac waveforms. The present research examined the value of the RZ interval (RZ), which has recently been proposed as a measure of sympathetic activity, for effort research. Also known as the initial systolic time interval (ISTI), RZ is the time (in ms) between the ECG R peak and the dZ/dt Z peak. Unlike PEP, RZ involves salient waveform points that are easily and reliably identified. Data from three experiments evaluated the suitability of RZ for effort research and compared it to a popular, automated PEP method. Participants completed a standard mental effort task in which correct responses earned a small amount of money. As expected, incentives significantly affected PEP and RZ in all three experiments. PEP and RZ were highly correlated (all rs ≥ .89), and RZ consistently yielded a larger effect size than PEP. A quantitative synthesis of the experiments indicated that the effect size of RZ’s response to incentives (Hedges’s g = .387 [.248, .527]) was roughly 20% larger than PEP’s effect size (g = .323 [.185, .461]). RZ thus appears promising for future research on sympathetic aspects of effort-related cardiac activity.