The sequence of ventricular contraction was studied by radionuclide phase imaging in 25 patients with Wolff-Parkinson-White syndrome. The studies were performed when no signs of precontraction were present in the electrocardiogram; in these cases pre-excitation was either intermittent or suppressable by injection of ajmaline. In 11 of the 16 patients with free wall accessory pathways, precontraction could be detected in spite of electrocardiographically absent pre-excitation. Discrete precontraction was seen also in 2 of the 9 patients with paraseptal accessory pathways. We conclude that antegrade conduction through the accessory pathway does not need to be completely blocked if signs of pre-excitation are absent on the electrocardiogram, and that phase imaging is, at least in some patients (especially those with free wall accessory pathways), a more sensitive technique for detection of pre-excitation (precontraction) than the electrocardiogram.