In 45 patients (15 female, 30 male; age 34 +/- 12 years) with Wolff-Parkinson-White syndrome: 1) the relation between electrophysiologic properties and location of accessory pathways and 2) the relationship between electrophysiologic properties of accessory pathways and adjacent atrial and ventricular myocardium was studied. Location of the accessory pathways was determined by catheter mapping of the coronary sinus and the tricuspid valve ring. There was no linear correlation between antegrade and retrograde effective refractory periods of accessory pathways and adjacent myocardial tissue. According to their location, accessory pathways were divided into right lateral (n = 4), anteroseptal (n = 6), posteroseptal (n = 10), left posterolateral (n = 8), and left lateral (n = 17). While analysis of variance revealed no differences between subgroups concerning retrograde effective refractory periods, antegrade effective refractory periods were significantly different (p less than .01). Moving in a clockwise direction around the mitral valve ring, antegrade effective refractory periods of the accessory pathways decrease from anteroseptal (321 +/- 30 ms) to posteroseptal (290 +/- 38 ms), left posterolateral (258 +/- 21 ms) and left lateral (246 +/- 27 ms), (Spearman R = 0.70m, p less than .01). Antegrade effective refractory periods of septally located accessory pathways (301 +/- 38) were significantly longer than of pathways located in the free wall of the ventricles (251 +/- 24, p less than .01).