The intensity of the proton magnetic resonance signal of the (CH2)n chain in phospholipids of sonicated lecithins is sensitive to the cholesterol content in the resulting vesicles. In the present study this signal has been used to monitor transfer of cholesterol between phospholipid vesicles. Vesicles prepared from pure egg yolk lecithin were mixed with vesicles that contained equimolar amounts of cholesterol and lecithin, and the time evolution of the (CH2)n signal intensity was followed. The results show that a homogenous distribution of cholesterol amoung vesicles is reached after about 4 h at 37 degrees C and 60 h at 4 degrees C. In order to determine the mechanism of the cholesterol transfer process, experiments were performed over a 2.5-fold range of vesicles concentrations. The accuracy of the kinetic results was not sufficient however to decide on the order of the reaction with respect ot vesicle concentration. Simultaneous observation of the choline proton resonance in the presence of Eu+3 and Pr+3 indicates that fusion between vesicles does not occur during cholesterol transfer.