Different equilibrium and non-equilibrium models are used to simulate drug transport and drug distribution. The percentage of absorbed drug, the rate constants of drug absorption and the drug concentrations in the different compartments of the models can be described quantitatively by the bilinear model, e.g., log ci = a log P-b log (betaP + 1) + c. A nearly perfect fit is obtained for the simulated data from this model. Drug absorption and distribution in biological systems can be explained and described by the model-derived equations. Examples from the literature include buccal absorption, gastric and intestinal in situ and in vitro absorption, colonic absorption, renal clearance, and absorption through the skin and the blood-brain barrier; in all those cases the bilinear model gives an excellent fit of the experimental data. Combination of the pH-partition theory with the bilinear model leads to a simple quantitative model for the precise description of the relationships between lipophilicity, degree of ionization, and absorption, distribution and biological activity of drugs.