Atrial conduction time (ACT) between a stimulating pacemaker electrode and an "impulse-detecting" monophasic action potential (MAP) electrode has been determined in 20 healthy males. A small increase in ACT was found with increasing paced heart rates. Using the extra stimulus technique, ACT was found to increase when the early ectopics approached the atrial effective refractory period (AERP). The degree of ACT prolongation was similar at long and short basic ACTs, indicating that the slowing of impulse propagation probably occurred near the simulating electrode and presumably within 1/2-1 cm from that electrode. The increase in ACT close to the AERP was of importance when determining the atrioventricular refractory period. In three recordings a supernormal phase of conduction was found. The small errors of the MAP recordings in detecting the time of excitation make the method sensitive enough to detect small differences in ACT.