Tissue:plasma (Pt:p) partition coefficients (PCs) are important parameters describing tissue distribution of drugs. The ultimate goal in early drug discovery is to develop and validate in silico methods for predicting a priori the Pt:p for each new drug candidate. In this context, tissue composition‐based equations have recently been developed and validated for predicting a priori the non‐adipose and adipose Pt:p for neutral organic solvents and pollutants. For ionizable drugs that bind to different degrees to common plasma proteins, only their non‐adipose Pt:p values have been predicted with these equations. The only compound‐dependent input parameters for these equations are the lipophilicity parameter, such as olive oil–water PC (Kvo:w) or n‐octanol–water PC (Po:w), and/or unbound fraction in plasma (fup) determined under in vitro conditions. Tissue composition‐based equations could potentially also be used to predict adipose tissue–plasma PCs (Pat:p) for ionized drugs. The main objective of the present study was to modify these equations for predicting in vivoPat:p (white fat) for 14 structurally unrelated ionized drugs that bind substantially to plasma macromolecules in rats, rabbits, or humans. The second objective was to verify whether Kvo:w or Po:w provides more accurate predictions of in vivoPat:p (i.e., to verify whether olive oil or n‐octanol is the better surrogate for lipids in adipose tissue). The second objective was supported by comparing in vitro data on Pat:p with those on olive oil–plasma PC (Kvo:p) for five drugs. Furthermore, in vivoPat:p was not only predicted from Kvo:w and Po:w of the non‐ionized species, but also from K*vo:w and P*o:w, taking into account the ionized species in addition. The Pat:p predicted from K*vo:w, P*o:w, and Po:w differ from the in vivoPat:p by an average factor of 1.17 (SD = 0.44, r = 0.95), 15.0 (SD = 15.7, r = 0.59), and 40.7 (SD = 57.2, r = 0.33), respectively. The in vitro values of Kvo:p differ from those of Pat:p by an average factor of 0.86 (SD = 0.16, r = 0.99, n = 5). The results demonstrate that (i) the equation using only data on fup as input and olive oil as lipophilicity surrogate is able to provide accurate predictions of in vivoPat:p, and (ii) olive oil is a better surrogate of the adipose tissue lipids than n‐octanol. The present study is an innovative method for predicting in vivo fat partitioning of drugs in mammals. © 2001 Wiley‐Liss, Inc. and the American Pharmaceutical Association J Pharm Sci 90:436–447, 2001