The microsomal metabolism of a variety of polycyclic systems has been investigated as part of an interest in structure-activity relationships governing cytochrome P-450 mediated bioactivation of polycyclic aromatic hydrocarbons (PAH). Considerable effort has recently been devoted to non-alternate PAH containing a fused five-membered ring, since many compounds in this class, such as cyclopenta (cd) pyrene and the benzofluoranthenes, are both mutagenic and carcinogenic. The authors report on the metabolism of a series of four cyclopenta-PAH isomers formed from benz(a)anthracene by fusing a five-membered ring between C4-C5 (benz(k)acephenanthrylene, B(k)A), C6-C7 (benz(e)aceanthrylene, B(e)A), C7-C8 (benz(j)aceanthrylene, B(j)A), and C11-C12 (benz(l)aceanthrylene, B(1)A). The metabolite profiles of these PAHs have been determined with Aroclor-1254, 5,6-benzoflavone and phenobarbital induced rat liver microsomes and the major metabolites unequivocally identified. In accord with the expected reactivity of the cyclopenta double bond towards epoxidation, the corresponding dihydrodiols are major metabolites of each of the PAHs. For B(l)A, and B(j)A, metabolites at the K-region are also observed. For B(e)A, formation of the 9,10-dihydrodiol (distal to the pseudo bay formed by the cyclopenta ring) is another major pathway. The mutagenicity of these PAHs is described in the following abstract.