Female F344/NCr rats were exposed continuously to Aroclor 1254 (1, 3.3, 10, 33, or 100 ppm in the diet) for 7, 28, or 84 days in order to assess the accumulation of polychlorinated biphenyls (PCBs) in liver, blood, and adipose tissue. The persistence of the individual PCB congeners which are detected in liver was examined in the three tissues of additional groups of rats exposed for 7 days followed by 21 days on control diet, or for 28 days followed by 56 days on control diet. Limited accumulation of PCB congeners with low chlorine substitution (tri- and tetrachlorobiphenyls) in the liver and blood, and preferential retention of highly substituted PCB congeners (penta- and hexachlorobiphenyls) were observed in rats continuously exposed to Aroclor. In these rats, time- and dose-dependent increases in the relative levels of two congeners which cause profound phenobarbital-type induction [2,2',3,4,4',5'-hexachlorobiphenyl (BZ# 138) and 2,2',4,4',5,5'-hexachlorobiphenyl (BZ# 153)] were detected in the liver and adipose tissue. Rats receiving control diet following Aroclor treatment displayed a time- and dose-dependent decrease in the relative levels in blood, adipose and hepatic tissue of 2,3,3',4,4'-pentachlorobiphenyl (BZ# 105) and 2,3',4,4',5-pentachlorobiphenyl (BZ# 118), two of the major congeners showing both TCDD- and phenobarbital-type induction. These rats also displayed increases in the relative adipose levels of another relatively potent mixed-type inducer, 2,3,3',4,4',5-hexachlorobiphenyl (BZ# 156), and increases in adipose and hepatic levels of the pure phenobarbital-type inducer, 2,2',4,4',5-pentachlorobiphenyl (BZ# 99).