[en] The photoaffinity labeling of the nuclear aryl hydrocarbon (Ah) receptor from mouse Hepa 1c1c7, rat hepatoma H-r-Ii-E and human Hep G2 cells using [³H]-TCDD as the photoligand was investigated. The cells were incubated with [³H]-TCDD in the presence or absence of actinomycin D and a 200-fold excess of 2,3,7,8-TCDF and after 3 hours the nuclear extracts were isolated and irradiated for 5 minutes (>300 nm). The yields of specifically-bound photocovalent adducts from the Hep G2, H-4-II-E and Hepa 1c1c7 cells were 56 ± 10.8, 48 ± 12 and 36 ± 1%, respectively. Comparable yields were observed for the photolysis of nuclear extracts from cells treated with actinomycin D. SDS-PAGE analysis of the photocovalent nuclear Ah receptor adducts from Hep G2, H-4-II-E and Hepa 1c1c7 cells gave a single specifically-bound band with apparent molecular weights of 115-, 100- and 95 kDa, respectively, and these results correspond with photoadducted cytosolic Ah receptor adducts. The in situ photolabeling of the nuclear Ah receptor in Hepa 1c1c7 cells was also investigated. The results showed that after a 3-minute irradiation of cell suspensions treated with [³H]-TCDD for 3 hours, the yield of nuclear Ah receptor photoadducts was 85.3 ± 2%. The results and applications of these photoaffinity labeling studies will be discussed