Left ventricular (LV) output and function was investigated, using pulsed Doppler echocardiography in 52 patients with various localizations of accessory AV pathway (AP) in sinus rhythm and during paroxysm of AV reentrant tachycardia. In patients with sinus rhythm and ventricular preexcitation the most marked decrease in LV output and function (reduced aortic flow peak velocity [PV], mean acceleration [MA], stroke distance [SD], minute distance [MD], and lengthened time to peak velocity [TTP]) was noted in the presence of right parietal AP and less marked changes (decreased MA, lengthened TTP)--in the presence of posteroseptal AP, in comparison with the controls (P less than 0.005). During antidromic tachycardia the pronounced decrease in PV, MA, SD, and MD was noted, especially in patients with left parietal and posteroseptal AP, while in the case of right parietal AP changes in MA and MD were insignificant. During orthodromic tachycardia the decrease in LV function was less marked and no significant differences in the magnitude of LV output and function changes were found in various localizations of AP, except MA, which was more severely decreased in patients with posteroseptal AP. We conclude, that the alterations in Doppler indices of LV output and function are related to the localization of AP during AV reentrant tachycardia and regular sinus rhythm.