Tacrine (1,2,3,4-tetrahydro-9-acridinamine monohydrate) is an inhibitor of acetylcholinesterase currently used in the treatment of the symptoms of Alzheimer's disease. The present study demonstrates preferential binding of this drug to acidic phospholipids, as revealed by fluorescence polarization, penetration into lipid monolayers, and effects on the thermal phase behavior of dimyristoyl phosphatidic acid (DMPA). A fivefold enhancement in the polarization of tacrine emission is evident above the main phase transition temperature (T(m)) of DMPA vesicles, whereas below T(m) only a 0.75-fold increase is observed. In contrast, the binding of tacrine to another acidic phospholipid, dimyristoylphosphatidylglycerol, did not exhibit strong dependence on T(m). In accordance with the electrostatic nature of the membrane association of tacrine, the extent of binding was augmented with increasing contents of egg PG in phosphatidylcholine liposomes. Furthermore, [NaCl] > 50 mM dissociates tacrine (albeit incompletely) from the liposomes composed of acidic phospholipids. Inclusion of the cationic amphiphile sphingosine in egg PG vesicles decreased the membrane association of tacrine until at 1:1 sphingosine: egg PG stoichiometry binding was no longer evident. Tacrine also penetrated into egg PG but not into egg PC monolayers. Together with broadening of the main transition and causing a shoulder on its high temperature side, the binding of tacrine to DMPA liposomes results in a concentration-dependent reduction both in the combined enthalpy delta H of the above overlapping endotherms and the main transition temperature T(m). Interestingly, these changes in the thermal phase behavior of DMPA as a function of the content of the drug in vesicles were strongly nonlinear. More specifically, upon increasing [tacrine], T(m) exhibited stepwise decrements. Simultaneously, sharp minima in delta H were observed at drug:lipid stoichiometries of approximately 2:100 and 25:100, whereas a sharp maximum in delta H was evident at 18:100. The above results are in keeping with tacrine causing phase separation processes in the bilayer and may also relate to microscopic drug-induced ordering processes within the membrane.