||The rat liver cytosolic receptor protein containing the Ah-receptor protein was purified and studied using a photochemical assembly of 2,3,7,8-TCDD. The unbound receptor protein rapidly lost its capacity to bind 2,3,7,8-TCDD; however, the 2,3,7,8-TCDD bound Ah receptor did not readily dissociate, probably reflecting the high potency and persistence of the toxicity of 2,3,7,8-TCDD. Results are based on a new one-step methodology which allows activation parameters to be calculated directly from raw experimental measurements, which allows the uncertainty in the activation enthalpy to be obtained unambiguously. The enthalpies of activation for both the formation and the interaction of the receptor-ligand complex are the same within the statistical uncertainty. This led to a kinetic model in which the receptor was activated to an intermediate followed by competitive degradation of the unoccupied receptor and formation of the receptor-ligand complex, both of these latter steps being fast compared with the first.