To evaluate the influence of atropine on atrial refractoriness and its dispersion, we studied ten subjects with sinus bradycardia who were otherwise healthy. Effective and functional refractory periods were measured at three sites of the right atrium (high, middle, and low in the lateral wall), in sinus rhythm and during atrial pacing (120/min), before and after i.v. administration of 0.04 mg/kg of atropine. Both before and after administration, dispersion of atrial refractoriness was determined from the range of refractory periods measured at the three atrial sites as the longest minus the shortest refractory period. Our data indicate that atropine was able to significantly reduce refractoriness and its dispersion. The study protocol allowed us to exclude the possibility that cycle length played a role. The antivagal effect of atropine seemed to explain our findings, even if the possibility that the drug had a direct effect could not be excluded.