The relationship between the volume of distribution, delta', extent of protein binding, EPB, and the apparent partition coefficient, APC, was investigated using 125 different drugs for which the data were retrieved from the literature, calculated, or determined experimentally. Equations were developed which allow one to predict the blood level concentration of a drug knowing the EPB, APC, and certain pharmacokinetic rate constants. Examples from the literature were cited in order to verify these equations. The delta-APC-EPB relationship can be used to estimate a drug's volume of distribution in a patient from in vitro data, and estimate the extent of absorption of extravascularly given drugs when I.V. data cannot be obtained.