Discrepancies between calorimetric and fluorescence depolarization monitoring of the pretransition in multilamellar vesicles of synthetic phosphatidylcholines are shown to result primarily from the slow rate of this transition. The depolarization of fluorescence of the membrane-associated dye 1,6-diphenyl-1,3,5-hexatriene was used to determine the temperature of the pretransition for a series of heating and cooling scan rates. These temperatures, when plotted vs. scan rate, extrapolated linearly to the transition temperature at zero-scan rate, Tm = 29.8 +/- 0.8 degrees C. The slopes obtained from these plots yielded characteristic times for the transition of 8 to 30 min. In addition, analysis of temperature-jump experiments, assuming first-order kinetics, gave characteristic times in the range 4--8 min. The data are taken to suggest a most likely value for the pretransition characteristic time of 5 +/- 2 min, with larger values possibly explainable by supercooling effects. Slight differences between the calorimetrically and fluorimetrically determined main transition temperatures appear to result from perturbation of the phosphatidylcholine bilayer by the fluorescent probe.