Ultrafast time-resolved pump-probe spectroscopy was studied to clarify the detailed mechanism in the photodissociation process of oxyhemoglobin in the visible spectral range. The photodissociation had not been time-resolved and only the upper limit of the time needed for the dissociation process was claimed to be faster than 50 fs; it was time-resolved to be 45 +/- 5 fs with 10 fs time resolution. The broadband spectrum of the visible laser pulses enabled us to observe the signal over a broad spectral range. A broadband multichannel detector array was used to simultaneously obtain the pump-probe signal at all probe frequencies. Thus, we obtained for the first time the spectra of the ultrashort lifetime electronic excited state of HbO(2) in the visible range from 523 nm (19 109 cm(-1)) to 719 nm (13 914 cm(-1)). After the photodissociation, subpicosecond time constant was found and attributed to the dynamics of the photolyzed Hb. Time-resolved difference absorption spectra in 0-100 fs showed oscillatory motion reflecting wavepacket motion in the potential energy surface of the photoexcited HbO(2) during the ultrafast photolysis.