The rate of turnover of the Ah receptor (AhR) was determined using the density shift method in Hepa 1 and in a Hepa 1 mutant line, c4, which fails to accumulate AhR complexes in the nucleus. The half-life of the AhR was found to be 7.7 and 9.7 h in Hepa 1 and c4 cells, respectively. The effect of AhR occupation with either an agonist, beta-naphthoflavone (beta NF), or a partial antagonist, alpha-naphthoflavone (alpha NF), on AhR half-life and concentration in the cytosolic fraction was examined. In Hepa 1 cells, a 12-h exposure to beta NF resulted in a 62% decrease in AhR concentration. The same treatment, using alpha NF as the ligand, resulted in a 14% decrease. The half-life of the AhR increased from 7.7 to 9.3 h during beta NF treatment and was essentially the same during alpha NF treatment in Hepa 1 cells. In c4 cells, a 12-h exposure to beta NF resulted in a 44% decrease in AhR concentrations, whereas exposure to alpha NF resulted in an 8% decrease. The half-life of the AhR in c4 cells during beta NF exposure increased from 9.7 to 14.6 h, and alpha NF exposure increased half-life to 17.6 h. These results indicate: (a) cytosolic AhR concentrations are modulated by ligand occupation, (b) exposure to AhR ligands, after an initial decrease in AhR levels, resulted in an increase in AhR half-life, and (c) similar results were obtained in Hepa 1 and c4 cells, this would indicate that AhR occupation with ligand and subsequent AhR-ligand nuclear translocation does not appear to play a significant role in regulation of AhR half-life in Hepa 1 cells.