Heart rate variability (HRV) indices that reflect the magnitude of respiratory sinus arrhythmia (RSA) are commonly applied as non-invasive measures of cardiac vagal control. Recently, however, serious doubts have been raised about the accuracy and validity of such assessments. To evaluate these methods, we derived a theoretical model for the dependence of mean heart rate and RSA on gradual vagal blockade by atropine, and compared its predictions to actual experimental results. The experiment involved the injection of nine consecutive intravenous bolus doses of atropine to eight young healthy male subjects. Seven-minute recordings of ECG and respiration were made for each atropine dose. The heart rate (HR) signal was derived from the ECG recording, and mean heart rate and the power of the high frequency peak of HRV (which measures the magnitude of RSA) were computed. The experimental data were fitted to the model's equations, and optimal values were obtained for the model's parameters. A tight agreement is observed between the theoretical fitted curves and the experimental data. The parameters that were computed from fitting the experimental data to the mean heart rate equation display a surprisingly small variance among the different subjects. The parameters that were computed from fitting the experimental data to the RSA equation, and the resulting shape of these fitted curves, explain many of the conflicting results previously published, and provide a new quantitative insight to cardiac vagal activity.