The influence of Aroclor 1254 (ARO) treatment or pair‐feeding (PF) on gluconeogen‐esis and urea synthesis and on isolated hepatic mitochondria was studied in rats of different ages. ARO (300 mg/kg, po on 4 consecutive days) induced variable weight loss in young (153 ± 10 g (initial wt), ‐10.9%), intermediate‐age (195 ± 10 g, ‐17.0%), and old (232 ± 23 g, ‐4.9%) rats. Isolated mitochondria contained equal amounts of cytochromes aa3,b,c1and c with exception that c1and c were lower in the young ARO rats than in the PF controls. Mitochondria from ARO rats, which lost more weight than ad libitum‐fed (AF) rats, showed suppression of ADP‐stimulated H+and oxygen uptake and succinate plus valinomycin maximal swelling in a potassium acetate and sucrose medium. Mitochondria from young ARO rats absorbed less incident light than mitochondria from PF or AF rats. Maximally swollen mitochondria from intermediate‐age ARO rats, contracted more rapidly with antimycin addition than those from PF or AF controls. These findings showed greater permeability of ARO mitochondria to impermeable and accumulated ions. In contrast, mitochondria from ARO rats without significant weight loss showed activation of ADP‐stimulated H+and oxygen uptake and maximal swelling in comparison to mitochondria from AF and PF rats, but contracted like these controls after the antimycin addition.