A re-evaluation of the dielectric studies on isolated mitochondria (Pauly, H., Packer, L. and Schwan, H.P. (1960) J. Biophys. Biochem. Cytol. 7, 589-601, and ibid. 7, 603-612) is presented. The suspensions of 'mitoplasts' prepared from rat liver mitochondria by a hyposmotic (10 mM KCl) treatment showed a dielectric dispersion with its characteristic frequency lying in the 1-100 MHz range. In the analysis of data special emphasis was put on the choice of the theoretical models to employ after scrutiny of their applicability to the suspensions tested. As such we adopted the theory of Hanai et al. (Hanai, T., Asami, K., and Koizumi, N. (1979) Bull. Inst. Chem. Res., Kyoto Univ. 57, 297-305) that was advanced to include concentrated suspensions of shelled spheres. Curve fittings based on that theory resulted in a better agreement with experiment than the fittings based on a conventional theory for dilute suspensions. Major findings from our analyses on the swollen mitoplasts are that: (i) the limiting membrane of the mitoplasts has a specific electrical capacity of 1 microF/cm2, (ii) the ratio of permittivity (or dielectric constant) for the mitoplast interior and permittivity for the external medium is 0.6-0.7, and (iii) the conductivity ratio between the interior phase and the medium is approx. 0.6. Reasons for discrepancy between the results of Pauly et al. and ours are discussed.