Ninety-six sexually mature male Wistar rats were divided randomly into four groups of twenty-four rats. In experiments to assess effects of sodium fluoride and sulfur dioxide on their sperm motility and serum testosterone (T), one group of rats was left untreated as controls, and the other three groups were administered, respectively, for eight consecutive weeks, 100 mg NaF/L (45 mg F – /L) in their drinking water, sulfur dioxide in ambient air (15 ppm SO 2 , 4 hr/day), or were exposed to both NaF and SO 2 together. In comparison with the control group, the ratio of testis weight to body weight increased in the NaF+SO 2 group at week 4, and the mean values of sperm motility were significantly lower in the NaF group and in the SO 2 group, and even more so in the NaF+SO 2 group over the entire eight-week period. The serum T level in the NaF+SO 2 group was significantly increased at week 2 and then markedly decreased at week 4 and 8. Similar changes were also observed in the SO 2 group. Sperm motility and serum T in rats were thus affected to some extent by NaF or SO 2 but were most drastically affected by NaF and SO 2 together. The resulting changes in the testis tissue and serum T concentration may therefore be connected with the low sperm motility in these rats.