Thioarenes, sulfur-containing polycyclic aromatic hydrocarbons, have been detected in a number of environmental sources. The metabolism of one thioarene, benzo[b]naphtho[2,1,d]thiophene ([2,1]BNT), by F344 rat liver 9000g supernatant (S-9) was studied. [2,1]BNT which is structurally analogous and has similar carcinogenic potency to chrysene, was metabolized to six ethyl acetateextractable metabolites when incubated with S-9 from Aroclor 1254-treated F344 rats. Each metabolite was collected from reverse-phase HPLC, and their identities were determined by analysis of MS and NMR data. In order of elution from HPLC they are as follows: (1) trans-1,2-dihydroxy-1,2-dihydrobenzo[b]naphtho[2,1-d]thiophene, (2) benzo[b]naphtho[2,1-d]-thiophene sulfone, (3) benzo[b]naphtho[2,1-d]thiophene sulfoxide, (4) trans-3,4-dihydroxy-3,4-dihydrobenzo[b]naphtho[2,1-d]thiophene, (5) 8-or 9-hydroxybenzo[b]naphtho[2,1-d]thiophene, and (6) 7-hydroxybenzo[b]naphtho[2,1-d]thiophene. In addition, the identities of metabolites 1, 2, 3, and 4 were confirmed by comparison to standards. The syntheses of the sulfone and sulfoxide of [2,1]BNT are reported here. The syntheses of the dihydrodiols were reported previously. Metabolite 5, a hydroxy[2,1]BNT, was the major metabolite formed by liver S-9 from untreated F344 rats. Microsomal preparations from these rats also produced significant amounts of the dihydrodiols, 1 and 4, and the sulfoxide, 3. Microsomes prepared from Wistar rats produced dihydrodiols and the sulfone and sulfoxide of [2,1]BNT. Therefore, [2,1]BNT is metabolized by both ring oxidation and sulfur oxidation in these two strains of rats.