Slow intramolecular mobility of native and inactivated actin from rabbit skeletal muscle during the process of protein unfolding induced by GdnHCl was studied using tryptophan room temperature phosphorescence (RTP). By this method, the conclusion was confirmed that an essentially unfolded intermediate preceded the formation of inactivated actin [Turoverov et al. Biochemistry (2002) 41, 1014-1019]. It was found that the kinetic intermediate generated at the early stage of protein denaturation has no tryptophan RTP, suggesting the high lability of its structure. Symbate changes of integral intensity and the mean lifetime of RTP during the U* --> I transition suggests a gradual increase of the number of monomers incorporated in the associate (U* --> I(1)... --> I(n)... --> I(15)), which is accompanied by an increase of structural rigidity. The rate of inactivated actin formation (I identical with I(15)) is shown to increase with the increase of protein concentration. It is shown that, no matter what the means of inactivation, actin transition to the inactivated state is accompanied by a significant increase of both integral intensity and the mean lifetime of RTP, suggesting that inactivated actin has a rigid structure.