Two types of hydration of bovine serum albumin, BSA, were found by microwave impedance dispersion according to Suzuhi et al. (J. Phys Chen r. 1996, 100, 7279). At 25.0 degreesC the number of the strongly bound water molecules, N-s, was 606 and that of the weakly bound water molecules, N-w, was 393 per one BSA molecule. Elevation of the temperature decreased N-s continuously, whereas it increased N-w discontinuously at 30 degreesC. Addition of an anesthetic halothane increased N, at the expense of N-w. At halothane 2.5 mM, N-s decreased by 3001 and N-W increased by about 350. The electrical conductance of BSA solution showed a minimum at about 1.0 mM halothane. The halothane concentration that showed the minimum electrical conductance coincided with the maximum in N-W. Because the volume fraction of BSA is less than 2.5%, the conductance change is attributable to the change of bulk ion concentrations. The initial decrease of conductance represents the suppression of ionization by halothane. The increase of conductance above 1.0 mM halothane indicates unfolding of the protein, exposing polar moieties to water, thus increasing the ion concentration.