Electrostatic potential maps of dibenzofuran, dibenzo-p-dioxin, 2,8-dichloro-3,7-dimethyldibenzo-p-dioxin, 2,3,7,8-tetrafluorodibenzo-p-dioxin, and 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) are presented and compared. The biological activities of the molecules increase from zero for the first two to very strong for the highly toxic TCDD. The analysis of the electrostatic potentials suggests certain key features that may be necessary for the effective interaction of the classes of compounds with a cytosolic receptor, which is believed to be the initial step leading to various toxic responses and other biological activity. (Copyright (c) 1986 American Chemical Society.)