The effects of Ca2+ and Mg2+ on the molecular motion of the polar head group in phosphatidylserine (PS) bilayers were measured by the time-resolved fluorescence depolarization method probed by 1,2-dihexadecanoyl-sn-glycero-3-phospho-[N-(4-nitrobenzo- 2-oxa-1,3-diazole)]ethanolamine [formula: see text] (NBD-PE). By this method, the rate and width of the molecular motion at the fluorescent moieties in the probe molecules could be evaluated as the wobbling diffusion rate (Dw, s-1) and the half cone angle of the wobbling cone (theta c, degree). The values of Dw and theta c measured for NBD-PE embedded in bovine brain phosphatidylserine bilayers were 3.7 x 10(7) s-1 and 46 degrees in the absence of divalent cations at 25 degrees C. When 3 mM of Ca2+ was added, both Dw and theta c distinctly dropped to 1.7 x 10(7) s-1 and 38 degrees, respectively. By the addition of 3 mM of Mg2+, however, only Dw decreased to 2.7 x 10(7) s-1 and theta c remained unchanged. These results show that both Ca2+ and Mg2+ decrease the rate of motion at the head part in PS molecules, but only Ca2+ narrows the distance between the neighboring head groups. Since Mg2+ does not promote vesicle fusion, it appears that the deformation at the head group region in the bilayer structure induced by Ca2+ is an important step in the membrane fusion process.