Phospholipids in biological membranes are arranged as bilayers. When constrained to pack into planar bilayers, certain phospholipids will form unstable structures as a consequence of their molecular shape and noncovalent bonding. This produces curvature strain which may provide energy for certain membrane processes. We demonstrate that an exothermic process associated with the relief of curvature strain can be detected calorimetrically. The enthalpy for the incorporation of a few percent lysophosphatidylcholine into large unilamellar vesicles of monomethyldioleoylphosphatidylethanolamine at pH 7.4 is exothermic but it is endothermic for stable bilayers such as this same lipid at pH 9 or dioleoylphosphatidylcholine at pH 7.4 or 9. The addition of lysophosphatidylcholine to monomethyldioleoylphosphatidylethanolamine at pH 7.4 is exothermic only for the addition of the first few percent of lysophosphatidylcholine and then it becomes endothermic. The size of the exothermic heat change is sensitive to changes in temperature, while the endothermic processes are relatively temperature-insensitive. The exothermic heat is also larger when 1 or 2 mol % of diolein is incorporated into vesicles of monomethyldioleoylphosphatidylethanolamine. These results are all consistent with the exothermic process corresponding to the relief of curvature strain in bilayers having a tendency to convert to the hexagonal phase. It provides a demonstration that considerable energy may be released upon the incorporation of certain molecules into membranes which have a low radius of spontaneous curvature.