In the present study the influence of toluene on the bending elastic modulus k(c), of dimyristoylphosphatidylcholine (DMPC) vesicles in the L-alpha phase is investigated using videomicroscopy. For giant unilamellar DMPC vesicles, a mean value of k(c) = (1.42 +/- 0.21) x 10(-19) J is determined at 30 degrees C. Addition of toluene, which is predominantly incorporated in the lipophilic core of the bilayer. induces a lowering of the bending elastic modulus with increasing concentration up to a plateau value of k(c) = (0.36 +/- 0.10) x 10(-19) J. This effect seems to be dominated by the mobility of the nonpolar molecules in the lipophilic part of the bilayer. Vesicles near the maximum solubilization capacity exhibit an anomalous increase of the bending elastic constant because of the formation of small hydrophobic dusters of phase-separated toluene. In the case of complete membrane saturation, these clusters can be clearly visualized in the vesicle membrane region. Thermally induced undulations vanish as a consequence of the reduction of the excess area a. Microscopy investigations of the temperature dependence of the bending elastic modulus were performed for both pure DMPC and toluene containing DMPC vesicles. Further analysis of the isobaric thermal area expansion coefficient beta indicates a reduced thermal expansivity for vesicles containing toluene. The values of beta vary from (6.9 +/- 0.8) x 10(-3) K-1 for DMPC vesicles to (1.6 +/- 0.3) x 10(-3) K-1 in the presence of toluene.