The dynamics of nitric oxide (NO) rebinding to ferrous myoglobin (Mb) and its model system, microperoxidase (Mp), in D2O solution at 283 K after photolysis was directly observed by monitoring the stretching mode of NO using femtosecond mid-IR absorption spectroscopy. The transient spectra of photolyzed MbNO reveal two overlapping vibrational bands for bound NO: 1598 (13.1 cm(-1) fwhm) and 1611 cm(-1) (12.7 cm(-1) fwhm). Time-resolved spectra of photolyzed MpNO are well described by a single vibrational band at 1658 cm(-1) (26 cm(-1) fwhm). Narrower vibrational bands within MbNO indicate that the protein environment encompassing bound NO is more homogeneous than the solvent environment of MpNO. The rebinding kinetics of photolyzed MbNO are the same for both vibrational bands, but are nonexponential and evolve according to the biexponental function: 0.54 exp(-t/5.3 ps) + 0.46 exp(-t/133 ps). In contrast, rebinding to MpNO is exponential with a time constant of 5.6 ps. MbNO exists in two distinct conformational substates that have the same geminate recombination kinetics. The ultrafast rebinding rate of MpNO indicates that rebinding of NO to the heme is almost barrierless. The similarity of the rebinding rate in MpNO to the faster phase of that in MbNO suggests that the slower phase of the rebinding in Mb be due to the protein environment surrounding the distal side of heme, and conformational relaxation of the protein after photolysis may raise the barrier of NO rebinding.