The apparent volume of distribution-after distribution equilibrium and the ratio of distributive tissue volume to the unbound fraction in the tissue (VT/fuT) of 10 weak basic drugs, i.e., chlorpromazine, imipramine, propranolol, disopyramide, lidocaine, quinidine, meperidine, pentazocine, chlorpheniramine, and methacyclin were compared in animal species and humans. In these two parameters, a statistically significant correlation between animals and humans was obtained, when the parameters were plotted on a log-log scale. The correlation coefficient between VT/fuT was significantly higher than that between the apparent volumes of distribution (p less than 0.05). In general, there was little difference between VT/fuT of various basic drugs in animals and that in humans. Prediction of the apparent volume of distribution in humans using animal data of VT/fuT, plasma unbound fraction, blood volume, and blood-to-plasma concentration ratio in humans was successful for most of drugs studied.