Natural background activity and food chain transfer of the uranium decay products, 210Po and 210Pb, were examined in the lichen-caribou-wolf food chain at two locations in the Northwest Territories of Canada. 210Po and 210Pb activities in lichens differed with species and location. Both 210Po and 210Pb were markedly higher in caribou bone than in wolf bone. 210Po activities in liver, kidney, and muscle were similar in both species. Caribou fetuses had lower activities of 210Po but higher activities of 210Pb than maternal muscle and placenta, suggesting greater placental transport of 210Pb than 210Po. Concentration ratios (CR = Bq kg-1 in consumer/Bq kg-1 in its food source) and ff values (ff in d kg-1 = Bq kg-1 in muscle/Bq d-1 ingested) showed that wolves retain more 210Po and less 210Pb from their diet than do caribou. 210Po CRs averaged 0.38 for caribou/lichens, 0.26 for caribou/rumen contents, and 0.40 for wolves/caribou. 210Pb CRs averaged 0.36 for caribou/lichens, 0.57 for caribou/rumen contents, and 0.13 for wolves/caribou.