The met and deoxy forms of sperm whale myoglobin (Mb) can be unfolded by guanidine hydrochloride (GuHCl). Electronic absorption and circular dichroism spectroscopic measurements show that folded deoxyMb is more stable than the folded met protein. Laser excitation of NADH generates species that rapidly reduce unfolded metMb, triggering the formation of folded deoxyMb in less than 10 ms (pH 7.0, 2.5 to 3 M GuHCl, 20 degrees C). At comparable reaction driving forces (similar to 10 kJ/mol), deoxyMb folds much faster than reduced cytochrome c.