The effects of spontaneous and atropine-induced variations of vagal tone on atrial refractoriness were studied in the conscious dog with complete atrioventricular block. Atrial rate and vagal tone, assessed with atropine, along with maximal paced atrial frequency were monitored for 90 days after the creation of atrioventricular block. The effects of atropine on maximal paced atrial frequency were also studied over the same period. Atrial rate was initially high and then fell back to values close to preoperative sinus levels. Vagal tone was first low and then rose back to values close to preoperative ones. The time-courses of atrial rate and vagal tone were correlated. The rise in vagal tone was accompanied by a correlated increase in maximal paced atrial frequency and thus a shortening of the atrial effective refractory period. However, atropine never significantly lengthened the atrial effective refractory period. This work clearly demonstrates the consistent shortening effect of vagal tone on the atrial effective refractory period in the conscious dog as well as the still unexplained inability of atropine to offset this effect, which may arise from a two-fold action of atropine on the heart.