1. Pigeons were injected with a single dose of commercial PCB mixtures (Aroclor 1248 plus Aroclor 1260), killed 120 h later and the abundance of individual PCBs was determined in adipose tissue, gonads, liver, brain, kidney, heart, muscle and blood. 2. Elimination factors for individual PCBs were calculated. Values of greater than 1 were obtained for PCBs with meta-para-unsubstituted carbon atoms in at least one ring, indicating that elimination exceeded accumulation in all or most tissues. By contrast, ortho-meta unsubstituted PCBs had elimination factors less than 1, thus indicating their impaired removal. 3. Tissues with high microsomal monooxygenase activity had the highest elimination factors for individual PCBs (i.e. liver greater than kidney greater than muscle greater than heart). 4. Distribution of individual PCBs was independent of sex and of ortho-chlorine substitution and showed that 90% of total PCBs in cadavers was present in adipose tissue, 2% in kidneys, 1% each in brain, muscle and heart and less than 0.1% in blood. 5. The distribution of the highly toxic non-ortho and mono-ortho substituted PCBs did not differ amongst all tissues analysed. 6. The present studies indicate that elimination of PCBs in vivo is favoured by the molecular feature of unsubstituted meta-para carbon atoms in the biphenyl moiety.