SummaryTo investigate the effect of restricted feeding (RF) on the absorption, metabolism, and accumulation of pentachlorobenzene (PECB), rats were fed either ad libitum (AD) or a restricted diet (at 25 or 50% of AD) for 15 days. On the 7th day (day 0), PECB (15mg/rat) was orally administered. Daily fecal weight and the total amount of PECB excreted into feces were decreased by RF, indicating an increase in PECB absorption due to its prolonged retention in the gastrointestinal tract. However, the amount of PECB excreted was only 4.8% of the dose given to the AD group, and 2.1 and 2.4% of that given to the 50 and 25% restricted diet groups, respectively. The concentrations of PECB and pentachlorophenol, and major metabolite, in blood of the restricted diet groups were higher on day 2 and day 4, while lower on day 6 and day 8 than those of the AD group. On day 8, PECB in liver, kidney, brain, and fat tissue were also lower in the restricted diet groups; the PECB levels in such tissue in the 25 and 50% restricted diet groups were 10-40 and 5-11% of that in the AD group, respectively. These results indicate that PECB metabolism was increased by RF. As PECB is lipophilic, it is most likely that the fat tissue mass, which was markedly decreased by RF, contributed to the enhancement of PECB metabolism. The mechanism seemed to be as follows: as fat tissue mass accumulating PECB decreased, the concentration of PECB in the liver increased, thus the amount of PECB metabolites formed was increased. Key Words pentachlorobenzene, absorption, accumulation, metabolism, fat tissue, food restriction, rat, pentachlorophenol Various organochlorine chemicals are present in the environment such as pesticides, herbicides, and intermediate products. These chemicals are lipophilic and tend to accumulate in fat tissue for a long time without being metabolized. The mobilization of such a chemical from the fat tissue into the blood is an important step toward its metabolism in the liver and its excretion from the body. Restricted feeding (RF) and fasting lead to decreased fat tissue mass, thereby mobilizing the chemical from the fat tissue together with lipolysis and enhancing its metabolism in the liver. In addition, it has been reported that RF treatment influences the activity of the hepatic drug-metabolizing enzymes (1, 2). The effects of RF on the excretion of DDT (3, 4), hexachlorobenzene (5, 6), and hexachlorobiphenyl (7, 8) have been tested. However, the results of these reports are inconsistent. In some cases, RF effectively enhanced chemical metabolism and excretion, but in other cases the treatment led to the redistribution of the chemicals, especially from fat tissue to the brain, and increased their toxicity. In most of these studies, the effect of RF was examined after the administration of the chemicals. RF prior to the administration of the chemicals might greatly influence both the transit time of the chemicals and fat tissue mass. The transit time might affect the absorption of the chemicals in the gastrointestinal tract,...