Semi-volatile lower chlorinated PCBs (LC-PCBs), comprised of mostly tetra or lower chlorinated congeners, are detected in old buildings and outdoors in high concentrations. PCBs cause endocrine disruption, neurotoxicity and many other adverse effects, and are human carcinogens. Epidemiological studies so far have relied upon serum concentrations of PCBs or OH-PCBs as markers of exposure. Despite the detection of LC-PCBs in high levels in buildings and other environments, only a few studies have reported LC-PCBs in human serum. One of the reasons for low serum detection of LC-PCBs could be the further biotransformation and excretion of OH-PCBs from the body. Therefore, the objective of this dissertation research was to study the metabolism of one of the LC-PCBs in rats and to identify a suitable metabolite marker of inhalation exposure for a future epidemiological study in humans. We chose PCB3 (4-chlorobiphenyl), a mono chlorinated PCB found in high concentrations in many buildings, as a model to study the metabolism. The first aim was to identify final metabolites of PCB3 in vivo. Male Sprague-Dawley rats were held in metabolism cages following exposure to PCB3 via i.p. injection. Blood, urine and feces were collected, and an analytical method for extraction of PCB3 metabolites was developed. By identifying the metabolites by LC/MS, a complete biotransformation pathway for PCB3 was elucidated. Major urinary metabolites of PCB3 were sulfates and mercapturates, while glucuronides and free phenolic forms were minor. The second aim was to study the disposition and toxicity of phenolic and sulfate metabolites after inhalation exposure to PCB3 in female Sprague-Dawley rats. Airborne PCB3 vapor was generated in a flask and passed through an inhalation exposure system, where rats were exposed via nose-only inhalation. Both hydroxylated and sulfated metabolites were detected in serum, liver, lungs, and brain. The serum clearance half-lives of these metabolites were less than two hours. Serum chemistry parameters were similar in PCB3 exposed and control rats. As a marker of bioactivation of PCB3 to electrophilic species, 8-oxo-dG was quantified in urine, but the difference was not statistically significant between control and exposed rats. The third aim was to study the routes of excretion of metabolites after inhalation to airborne PCB3. Bile cannulated and intact rats were exposed to PCB3 via nose only inhalation. Metabolites detected in bile were mostly sulfates and some glucuronide. Fecal metabolites were exclusively phenols despite very low concentration of free phenolic forms in bile. Peak excretion of metabolites in feces and urine occurred within 24 h, and over sixty percent of the dose was recovered within 24 h. The major route of excretion of PCB3 was urine in the form of sulfated metabolites. Sulfated metabolites were stable in urine for a month without aid of any preservatives. This dissertation shows that PCB3 is rapidly metabolized to phenols and conjugated mostly to sulfates. Conjugated metabolites, elaborated into bile, are either reabsorbed or hydrolyzed in the gut and excreted in feces as phenolic forms. Serum concentrations of sulfates are higher than free phenolic forms at any time after exposure, and are also detected in lungs and brain. PCB3 sulfates are stable metabolites and can serve as a metabolite marker of inhalation exposure to PCB3.