The effect of increasing concentrations of mequitazine, a quinuclidinylmethyl-phenothiazine, on the phase transition temperature (Tc), the broadening of the transition peak, the enthalpy and entropy of transition of dimyristoyl-, dipalmitoyl- and distearoylphosphatidylcholine (DPPC) liposomes was studied. Pest critical micelle concentrations of mequitazine (CMC = 5.23 X 10(-2) M), caused broadening of the transition peak and lowering of the Tc of pure liposomes. The ratio of peak heights from the nuclear magnetic resonance (NMR) spectra of egg phosphatidycholine liposomes was used as a criterion for assessing the interaction of the drug with phsopholipid membranes. Mequitazine interacts with both the polar head groups and hydrophobic membrane interior.