Electrophysiologic studies were conducted in 21 patients with sinus nodal dysfunction before and after intravenous administration of 1 to 2 mg of atropine. The mean sinus cycle length (+/- standard error of the mean) was 1,171 +/- 35 msec before and 806 +/- 29 msec after administration of atropine (P less than 0.001). Mean sinus nodal recovery time determined at a aced rate of 130/min and maximal recovery time were, respectively, 1,426 +/- 75 and 1,690 +/- 100 msec before and 1,169 +/- 90 and 1,311 +/- 111 msec after atropine (P less than 0.001 and less than 0.001). Mean calculated sinoatrial conduction time, measured in 16 patients, was 113 +/- 8 msec before and 105 +/- 9.7 msec after atropine (difference not significant). Mean atrial effective refractory period, measured at an equivalent driven cycle length, was 262 +/- 11.1 msec before and 256 +/- 10.3 msec after atropine (not significant). Mean atrial functional refractory period was 302 +/- 12.5 msec before and 295 +/- 11.3 msec after atropine (not significant). The shortening of sinus cycle length and sinus recovery time with atropine was similar to that noted in patients without sinus nodal dysfunction. In contrast, atropine had insignificant effects on sinoatrial conduction and atrial refractoriness in this group whereas it shortens both in normal subjects. This finding may reflect altered perinodal and atrial electrophysiologic properties in patients with sinus node disease.