The kinetics of Ca2+-induced fusion of phosphatidylcholine-phosphatidic acid vesicles has been studied using the dependence of proton nuclear magnetic resonance linewidths on vesicle size. The linewidth of the lipid acyl chain methylene resonance has been shown to be sensitive to changes in vesicle size but insensitive to vesicle aggregation. For vesicle systems with the same lipid composition, the linewidth increases in a linear fashion with vesicle radius over the range 125--300 A. This dependence has been used to determine quantitatively fusion rates and the dependence of such rates of Ca2+ as well as an vesicle concentration. For vesicle concentrations in the range of 3 . 10(-6)--10(-5) M and Ca2+ concentration at a level approaching 1 : 1 with respect to phosphatidic acid, the initial fusion rates have been found to be fast, with half-times of 1--10 min. An order of reaction of 2.7 with respect to vesicle concentration has been observed. Mechanisms of vesicle fusion are discussed in view of these observations.