The conditions of structural modifications of horse heart cytochrome c (pH, salt concentration) have been studied. Under these conditions the rate of carboxycytochrome c formation greatly increases in the course of the reduction process as compared to this rate after cytochrome c reduction and relaxation to the equilibrium state. According to these results the reduced intermediate which appears in the course of reduction has a high affinity for the carbon monoxide. It has been shown that the reduced low-spin cytochrome c practically does not take part in the process of dynamic conformational equilibrium with other cytochrome c forms existing in equilibrium mixture of oxidized and reduced cytochrome c.