1. Different levels of parasympathetic control of the heart were measured using ratios of the longest to the shortest intervals between R-waves in the ECGs (R-R) recorded during a deep breathing exercise in the supine position (respiratory sinus arrhythmia). The mean of absolute differences in consecutive R-R intervals was also measured in the supine position during quiet breathing (beat-to-beat variation), in six healthy volunteers, after cumulative doses of atropine (0.1-2.0 mg 70 kg-1, i.v.). 2. Full atropinization was observed at 2.0 mg 70 kg-1 where the respiratory sinus arrhythmia decreased by 97%, and the beat-to-beat variation dropped by 94%. There was no further drop in both indices at doses above 2.0 mg 70 kg-1. The different levels of parasympathetic control of the heart were directly proportional to the equivalent levels of respiratory sinus arrhythmia or beat-to-beat variation. 3. A universal linear vagal scale was calculated for use with these non-invasive indices to assess the levels of parasympathetic control of the heart. Zero in this scale would represent no activities in the cardiac vagus nerve. 4. It was concluded that vagal tone could be selectively measured in an intact person using these simplified and fast algorithms for quantifying respiratory sinus arrhythmia or beat-to-beat variation. A linear scale for measuring the vagal tone was possible. This linear scale was called the linear vagal scale (LVS).