It is known that when bilayers of some saturated phosphatidylcholines are stored for 3 or more days at similar to 0 degrees C, a lamellar subgel (L-c) phase is detected at temperatures below the pretransition by differential scanning calorimetry (DSC). However, the subgel (L-c) phase and the corresponding subtransition (L-c -> L-beta') for dimyristoylphosphatidylcholine ( DMPC) has not been clearly characterized. In this study, using the temperature jump protocol first developed by Tristram-Nagle et al. for the dipalmitoylphosphatidylcholine ( DPPC) system, new and accurate data characterizing the subgel formation and subtransition of DMPC were obtained through DSC and fluorescence spectroscopy with 1,6-diphenyl-1,3,5-hexatriene (DPH). It was discovered that the formation of the DMPC subgel phase requires incubation at temperatures of -5 degrees C or lower for 2 h or more. Kinetics of the subgel formation indicate that it is a very complex process and demonstrates that the planar gel phase is merely metastable below the subtransition, and not the thermodynamically stable phase. The subgel growth of DMPC is proven to be the dehydration of the headgroup region, and the subtransition is a process in which poorly hydrated DMPC becomes hydrated.