Spin-lattice relaxation times (T1) of intracellular water of the nonmyelinated fibers of rabbit cervical vagus nerve were measured using a paramagnetic shift reagent, s-FDF. Spin-lattice relaxation decay curves were composed of the fast (T1,F) and slow (T1,S) relaxation components. The mean values of T1,F and T1,S in the polarized nerve fibers at 25 degrees C were 0.12 +/- 0.04 and 0.61 +/- 0.13 sec., respectively, and their fractions were 0.71 +/- 0.07 and 0.29 +/- 0.07, respectively (n = 21). On the other hand, those values in the depolarized nerve fibers were 0.16 +/- 0.01 sec., 0.77 +/- 0.17 sec., 0.77 +/- 0.10 and 0.23 +/- 0.10, respectively (n = 15). T1,F and T1,S for the depolarized nerve fibers were significantly elongated (P < 0.01).