A study of atmospheric electric field measurements at the CN Communications Tower in Toronto, Canada (height 553 m above ground level (AGL)) has been conducted during the period from 1978 to 1983. Of particular interest were the field conditions during the times when lightning struck the tower. Measurements were carried out using fixed radioactive probes and a movable Stanford-University-type field mill located at 366 m AGL. Results show that the electric field measurements at 366 m AGL are significantly influenced by their azimuthal location under cloudy conditions and that approximately 23% of the lightning strikes to the tower occur in the absence of a strong vertical electric field.