Firefly luciferase is a soluble enzyme which is unusually sensitive to general anesthetics. The inhibition of the highly purified enzyme by three inhalational and three alcohol general anesthetics has been studied as a function of temperature, in the range from 5 to 20 degrees C. Inhibition constants Ki were determined at different temperatures, and van't Hoff plots of ln (Ki) versus reciprocal absolute temperature were found to be linear for all agents. Analysis of these plots gave values for the standard Gibbs free energy, enthalpy and entropy changes for transferring each anesthetic from water to the anesthetic-binding pocket on the protein. The most striking finding was that the enthalpy changes were much more negative for anesthetics binding to the protein than for binding to lipids or simple solvents. Furthermore, amongst the set of anesthetics studied, it was found that increasing potency correlated with favorable enthalpy rather than entropy changes. We discuss our results with respect to the molecular mechanisms underlying general anesthesia.