Cycloheximide superinduces the transcription of CYP1A1 in the presence of an agonist for the Ah receptor (AhR). To investigate the molecular target for "superinduction," we analyzed the agonist-induced degradation of AhR. Whereas 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD), a potent agonist of AhR, induces a rapid reduction of the AhR protein, cycloheximide blocks the downregulation of steady state AhR. Analyses of the turnover of AhR reveal that cycloheximide blocks the shortening of the half-life of AhR by TCDD, Blocking of the TCDD-induced AhR degradation requires inhibition of protein synthesis, because (a) cycloheximide inhibits protein synthesis at the concentration at which it causes superinduction and inhibition of AhR degradation; and (b) puromycin, an inhibitor of protein synthesis by mimicking aminoacyl-tRNA, also blocks the TCDD-induced AhR degradation. The blocking of the TCDD-induced AhR degradation correlates with the superinduction of CYP1A1 gene expression in a time- and dose-dependent manner. Furthermore, cycloheximide is shown to increase the accumulation of the TCDD-activated AhR and the functional AhR Amt complex in nucleus. Collectively, our results reveal a mechanism of superinduction by cycloheximide by enhancing the stability of agonist-activated AhR, The finding that inhibition of protein synthesis blocks the TCDD-induced AhR turnover implicates a cycloheximide-sensitive, labile factor (designated as (A) under bar hR (d) under bar egradation (p) under bar romoting (f) under bar actor, or ADPF) in controlling the removal of agonist-activated AhR in nucleus.