Diets that contained various levels of supplemental Aroclor 1242 or Aroclor 1016 were fed to mink and ferrets to investigate the chronic toxicity of these PCBs in two closely related species. In mink, Aroclor 1242 was found to be more toxic than comparable or higher levels of Aroclor 1016. The Aroclor 1242 diets caused complete reproductive failure at levels as low as five ppm of the diet. Aroclor 1016 impaired reproduction less than Aroclor 1242. Although fewer females whelped and the four-week kit weights were less than the control animals, no outward signs of abnormalities beyond their smaller size were found in the kits whelped and nursed by dams fed Aroclor 1016. Ferrets were more resistant to the effects of either PCB mixture than were the mink, as noted by the lower mortality rate on the Aroclor 1242 diet and the almost normal level of reproduction on the Aroclor 1016 diet. Feeding Aroclor 1242 at 20 ppm resulted in complete reproductive failure, but was not fatal to adult ferrets. This finding is in sharp contrast to the 100% mortality of adult mink fed the same level. Although the chlorine content is similar in both compounds, Aroclor 1242 has a higher percentage of molecules with five or more chlorines per biphenyl. This difference in higher substituted biphenyl isomer content and/or the reduced levels of contaminants in the Aroclor 1016 mixture may be of major importance in evaluating the toxicity of these compounds.