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Expanding on recently published methods, the current study presents an approach to estimating the dynamic, regulatory effect of the parasympathetic nervous system on heart period on a moment‐to‐moment basis. We estimated second‐to‐second variation in respiratory sinus arrhythmia (RSA) in order to estimate the contemporaneous and time‐lagged relationships among RSA, interbeat interval (IBI), and respiration rate via vector autoregression. Moreover, we modeled these relationships at lags of 1 s to 10 s, in order to evaluate the optimal latency for estimating dynamic RSA effects. The IBI (t) on RSA (t‐n) regression parameter was extracted from individual models as an operationalization of the regulatory effect of RSA on IBI—referred to as dynamic RSA (dRSA). Dynamic RSA positively correlated with standard averages of heart rate and negatively correlated with standard averages of RSA. We propose that dRSA reflects the active downregulation of heart period by the parasympathetic nervous system and thus represents a novel metric that provides incremental validity in the measurement of autonomic cardiac control—specifically, a method by which parasympathetic regulatory effects can be measured in process.