In preparation for a study of the relative oral bioavailability of polychlorinated dibenzo-p -dioxins and dibenzofurans (PCDD/Fs) in soils (typically containing less than 1 ppb 2,3,7,8-tetrachlorodibenzo-p-dioxin [TCDD] toxic equivalents [TEQ]), the background concentrations of PCDD/Fs and selected polychlorinated biphenyls (PCBs) were measured in liver and adipose tissue from female Sprague-Dawley rats and juvenile swine after 30 d of ingesting laboratory chow. The measured concentrations of TCDD and other PCDD/Fs in rat livers were severalfold less than previously reported in the literature for control (unexposed) laboratory rodents. The concentrations of PCDD/Fs and selected PCBs in livers of swine were three- to fourfold lower than those reported for rats. The lower concentrations found in this study compared to previous findings may be due to inadvertent laboratory contamination in previous studies or to declining levels of PCDD/Fs in laboratory feed, which parallel the declines in emissions, general environmental levels, and human food and tissue levels of PCDD/Fs.