The effect of parasympathetic blockade with intravenously administered atropine (0.04 mg/kg body weight) on the heart rate (HR) of 11 normal males was evaluated using a standardized isometric exercise (handgrip). This type of exercise, when maintained for 10 s at the maximum intensity tolerated by the volunteer, causes tachycardia mediated almost exclusively by parasympathetic withdrawal. The test was performed under baseline conditions and at 20 +/- 3, 33 +/- 3, 42 +/- 3, 52 +/- 3 and 61 +/- 3 min after atropine infusion. The heart rate of the subjects at rest in the supine position (mean +/- SEM) was 70 +/- 2 bpm, reaching values of 119 +/- 3 bpm after atropine. Under control conditions (sitting position), the increase in HR induced by isometric exercise was 22 +/- 1.8 bpm. After blockade, the increase was 4 +/- 0.8, 4 +/- 0.7, 5 +/- 0.6, 4 +/- 1.0 and 4 +/- 0.8 bpm, respectively, at each repetition of the test. These results suggest that the blockade induced by this dose of atropine is effective for at least 1 h. They also indicate the need for a more rigorous definition of the functional half-life of atropine in the heart, when considering physiological tests that either stimulate or withdraw parasympathetic activity upon the heart.