Using femtosecond vibrational spectroscopy, we investigated the rebinding dynamics of NO to cytochrome c (Cytc) and a model heme, microperoxidase-8 (Mp), after photodeligation of CytcNO in D2O solution and MpNO in an 81% glycerol/water (v/v) mixture at room temperature. Whereas the stretching mode of the NO band in MpNO was described by a Gaussian centered at 1653 cm(-1) with a full width at half-maximum (fwhm) of 41 that in CytcNO revealed an asymmetric structured band that peaked at 1619 cm(-1) with an fwhm of about 27 cm(-1). The structured NO band in CytcNO was well described by the sum of three Gaussians, and its shape did not evolve with time but its amplitude decayed exponentially with a time constant of 7 +/- 1 ps. The transient NO band in MpNO also decayed exponentially with a time constant of 8 +/- 1 ps. Rebinding of NO to Cytc was slightly faster than that of NO to Mp and was almost complete by 30 ps, which was much faster than the rebinding of NO to myoglobin (Mb). When the deligated NO was constrained near the Fe atom either by a viscous solvent or by the protein matrix, it rebound to heme Fe much faster than CO, suggesting that NO has a higher propensity for binding to heme Fe and the high reactivity governed,the rebinding kinetics. Moreover, the faster ligand rebinding in Cytc than in Mb suggests that Cytc does not have a primary docking site (PDS)-like structure found in Mb that suppresses rebinding by restraining ligand motion and the PDS can also hold the deligated NO in a manner that impedes NO rebinding; however, due to higher NO reactivity with heme Fe, the impediment is not as efficient as for CO.