The detailed mechanisms leading to soft tissue contrast in MRI are not known. To explore the physical basis for this phenomena, the effect of acyl chain packing of phospholipid model membranes on water proton relaxation were investigated at 20 MHz at 40 degrees C. Three variables affecting lipid packing were examined: chain composition, toluene concentration, and pH/ionic strength. It is demonstrated that the T2 of water protons is strongly influenced by changes in lipid packing, while T1 is generally insensitive to these variables. It is also shown that small changes in water content (7 wt%) altered both T1 and T2 in all systems studied. The results suggest that the physical properties of biological membranes in tissues are an important factor in determining T2.