Using a model consisting of Fe2+ octahedrally coordinated to five negative point charges and a CO molecule, the effects of a uniform external electric field applied along the Fe-C-O axis on the chemical shifts of C-13 and O-17 and the CO stretching frequency are investigated. Both the vibrational frequency and the O-17 chemical shift are observed to increase as the electric field applied parallel to the Fe-C-O axis is increased. On the other hand, the C-13 chemical shift changes in the opposite direction. The influence of a nonuniform field is also studied by introducing an additional variable point charge along the Fe-C-O axis. Results of the calculations agree well with observed correlations of C-13 and O-17 chemical shifts and CO vibrational frequencies in carbonmonoxy heme proteins.