With the help of PMO theory a non-empirical sector rule has been derived for the n ~ 1t5* band CD (around 260 nm) of the pentadienolide chromophore of steroidal bufadienolides. The rule was verified by experiment using the preferred conformation for the 2-pyrone side chain found in most of the crystalline bufadienolides investigated by x-ray diffraction. Substituents in ring D do not drastically influence the Cotton effect of this transition, whereas the sign of the 1C4 ~ 1t5* band Cotton effect around 300 nm is strongly dependent on such substitutions. The Cotton effect of additional carbonyl groups can in general be detected without difficulty in the CD-spectra
