Sample prepn. and ligand complex prepn. of Hb I from Lucina pectinata were performed to obtain ultrafast data on ferric HbI-NO, HbI-N3, HbI-H2S and metHbI complexes that suggest a mechanism for the photoinduced redn. of HbI species. Formation of a transient species in the 430 nm region of the ferric derivs., plus the fact that subsequent to the ultrafast measurements a ferrous oxy HbI species was obsd., led to the suggestion that probably a redn. of a ferric to a ferrous deriv. was occurring. Hb I photoredn. take place when the sample is exposed to a femtosecond laser source with power ranging from 0.3 mJ to 10 mJ, showing a population dependency with power intensities. The spectral band displacement obsd. in UV-Vis spectrum obtained after ultrafast expts. confirmed that a reduced species was formed, however, all species show similar dynamics which ends in 15 ps. When ferric samples are photoexcited in a CO atm., the reduced species formed is stable enough for CO to interact with the heme at longer time scales. The formation of the HbI-Co complex suggests that the photoinduced ferrous deriv. has similar structural and chem. properties to deoxyHbI since no difference was obsd. between the fully reduced by photoinduction and the chem. reduced HbI-CO complex spectra. The proposed process that is taking place after photoexcitation is described. [on SciFinder (R)]