Understanding the role of prostaglandin H synthase (PHS) in the carcinogenic process and the metabolic steps involved in the activation of carcinogens will facilitate experiments using pharmacological agents to prevent carcinogenesis. This study assesses the relative amounts of PHS-catalyzed binding of the urinary tract carcinogens [2-14C]-N-[4-(5-nitro-2-furyl)-2-thiazolyl] formamide (FANFT) and [2-14C]-2-amino-4-(5-nitro-2-furyl) thiazole (ANFT). Binding to protein and nucleic acid was assessed using PHS prepared from ram seminal vesicle, dog bladder transitional epithelium and rabbit renal inner medulla. PHS-catalyzed binding of ANFT was significantly greater than FANFT in each tissue. Substrate and inhibitor experiments were consistent with the prostaglandin hydroperoxidase activity of PHS catalyzing the binding of FANFT and ANFT. Oxygen was required for metabolism with arachidonic acid but not with peroxide as cosubstrate. The amount of PHS-catalyzed ANFT binding to protein was at least 4-fold greater than FANFT. Whereas a significant amount of FANFT was bound to protein, no FANFT binding to DNA could be detected. By contrast, PHS catalyzed the binding of ANFT to both protein and DNA. A PHS-catalyzed metabolite of ANFT was tentatively identified as the 4-keto analog by mass spectral analysis. The lower rate of PHS-catalyzed metabolism of FANFT compared to ANFT and the lack of detectable FANFT binding to DNA suggest that the metabolic steps involved in the initiation of FANFT-induced bladder cancer include 1) deformylation of FANFT to ANFT, 2) PHS-catalyzed activation of ANFT and 3) binding of an activated ANFT metabolite(s) to DNA.