The diol-epoxide r-7,t-8-dihydroxy-t-9,10-oxy-7,8,9,10-tetrahydrobenzo(a)pyrene is a potent mutagen and possibly the ultimate carcinogenic form of benzo(a)pyrene. A (7/8,9)-trihydroxy-7,8,9,10,10-pentahydrobenzo(a)pyrene is formed from the diol-epoxide r-7,t-8-dihydroxy-t-9,10-oxy-7,8,9,10-tetrahydroxybenzo(a)pyrene by reduction with NADPH. Its formation is linear with NADPH concentration and does not require the presence of enzyme. A (7,9/8)-trihydroxy-7,8,9,10,10-pentahydrobenzo(a)pyrene is similarly formed from the diol-epoxide r-7,t-8-dihydroxy-c-9,10-oxy-7,8,9,10-tetrahydrobenzo(a)pyrene by reduction with NADPH. The structures of the trihydroxypentahydrobenzo(a)pyrenes were established by their UV absorption and mass spectra and their reaction with potassium triacetylosmate.