The use of an isotonic, pH 7.4,1-octanol saturated phosphate buffer with added N,N-dimethyloctylamine (DMOA) at 1-4 mM on persilated RP-18 reversed-phase high-performance liquid chromatography can give excellent agreement (r > 0.99) with 1-octanol bulk phase shakeflask distribution coefficients for lipophilic amines, such as phenothiazines and tricyclic antidepressants. This system can be a superior model of biological partitioning compared to other reversed-phase high-performance liquid chromatography systems, e.g., 20%, v/v, CH3CN in the same buffer, usually with or without added DMOA. Requirements for an adequate data set are discussed. Histamine-releasing activity in rat mast cells for a series of 14 phenothiazines and tricyclic antidepressants is best correlated by this optimized system (r = 0.929) compared to the organic system on C-18 (r = 0.873). Addition of 4 mM DMOA to the organic system improves the correlation (r = 0.913); this may indicate that the mode of activity is nonspecific binding of the lipophilic amine to the mast cell. On the other hand, binding of seven phenothiazines to BSA was found to involve a specific interaction of the aliphatic nitrogen to the protein. Correlation using the optimized system with a +pKa term (r = 0.980) was superior to the other systems. The best organic modifier correlation was r = 0.939, without a pKa term. This could be interpreted as indicating that the latter system already contains a contribution from the basicity of the aliphatic nitrogen, which is supported by other evidence. Finally, the inhibition of (Na+,K+)ATPase by nine lipophilic amines was about equally well correlated by the optimized system as by the 20%, v/v, CH3CN system with added DMOA (r = 0.96). Omitting the DMOA from the organic system gave a poorer correlation (r = 0.911). This is consistent with the putative mechanism of action of ATPase, a membrane-bound enzyme. Binding of drug occurs to the membrane lipids, inducing a conformational change indirectly in the enzyme, which is then deactivated. Since the drug does not directly interact with the enzyme, there is less discrimination between the different partitioning systems. The apparent discrimination observed for the histamine release therefore requires further clarification. The use of several high-performance liquid chromatography systems can help clarify the quantitative structure-activity relationships.