We report electrostatic potential and electric field calculations at the CO ligand of the heme in both horse heart and yeast cytochrome c, obtained from a finite difference solution to the Poisson-Boltzmann equation. This method takes into account the protein shape and charge distribution, as well as the solvent and generalized ionic strength effects. The calculations support recent experimental and theoretical evidence suggesting that polar interactions can significantly affect the vibrational frequency of the ligand. Our work shows that the ionizable amino acid residues and polar contributions of the protein matrix can induce a Stark effect on the CO stretch frequency of the carbonmonoxycytochromes. The observed CO stretches, at 1965.9 and 1960.1 cm(-1) for horse and yeast cytochromes c, respectively, show that the experimental shift is of the order of 6 This is in good agreement with a calculated value of 8 cm(-1) (+/- cm(-1)) for a vibrational Stark shift due to the different charge distributions in both cytochromes.