X-ray absorption fine structure (XAFS) data were obtained from frozen aqueous solutions (10 K) of horse heart ferri- and ferrocyt c. Models of the structure about the Fe center were refined to optimize the fit between the observed XAFS in the range 0 less than or equal to k less than or equal to 16.3 Angstrom(-1) and the XAFS calculated using both single-scattering (SS) and multiple-scattering (MS) calculations. The bond lengths obtained are more accurate and precise than those determined previously for cyt c from various species using X-ray crystallography. The Fe-N bond lengths are 1.98-1.99 Angstrom for both oxidation states of cyt c. The Fe-S bond of ferricyt c (2.33 Angstrom) is significantly longer than that of ferrocyt c (2.29 Angstrom). The small changes in the bond lengths are consistent with the small reorganizational energy required for the fast electron-transfer reaction of cyt c.