The purpose of the present study was to investigate the effect of swimming on nitric oxide synthase expression in the hippocampal dentate gyrus of the Sprague-Dawley (SD) rats. The male SD rats weighing 250±10 g (6 weeks old) were used in the experiment. In the first part of the experiment (involving session duration-dependence swimming), the rats were divided into five groups to investigate an optimum duration of swimming exercise for the prevention of the NO-dependent neuronal damage: control group, 1-min-swimming group, 5-min-swimming group, 10-min-swimming group, and 20-min-swimming group (n = 10 in each group). In the second part of the experiment (involving training duration-dependence swimming), the rats were divided into five groups: control group to investigate an optimum program of swimming training for the prevention of the NO-dependent neuronal damage, I-day-swimming group, 3-day-swimming group, 7-day-swimming group, and 14-day-swimming group (n = 10 in each group). The rats in the session duration-dependence swimming groups were made to swim for 1 min, 5 min, 10 min, and 20 min once a day for 3 consecutive days, respectively. The rats in the program duration-dependence swimming groups were made to for 10 min once a day for 1 day, 3 days, 7 days, and 14 days, respectively. The swimming apparatus consisted of a swimming pool with 60 cm in height and 120 cm in diameter, which was filled with water at a temperature of 30-32℃ up to 45 cm from the bottom. With respect to the session-duration-dependence swimming groups. the two swimming groups of 10 min or longer duration significantly decreased the number of NADPH-d-positive cells in the dentate gyrus compared to either the control group (p = 0.001) or the I -min swimming group (p = 0.001) or the 5-min swimming group (p = ??). With respect to the program-duration swimming groups, all the swimming groups regardless of its training frequencies had significantly decreased NADPH-d-positive cells in the dentate gyrus compared to the control group (p = 0.03). In conclusion, these results of the current study suggest that a single bout of swimming lasting longer than 10 min, regardless of its frequency, may be an effective means for the prevention of NO-dependent neuronal damage.