Abstract: Objectives To observe the effects of arsenic exposure on learning and memory ability and the expression of C-Jun in the hippocampus of rats, in order to investigate the mechanism of learning and memory injury from arsenic exposure.Methods Forty SD rats (weight:100±20 g; half in male and female)were randomly divided into four groups, 10 rats in each group. The concentration of sodium arsenite in drinking water was 0, 2.4, 12 and 60 mg/L for each group for 100 days. Morris water maze was used for the evaluation of learning and memory ability. The level of arsenic in brain was detected with ICP-OES. The expression of C-Jun mRNA in hippocampus was measured by qRT-PCR technique. The expression of Jun protein in hippocampus was observed by immunohistochemistry.Results Arsenic level in the brain of arsenic exposure groups was significantly higher than the control group (P<0.05). The latent time to find out the hidden platform in a place navigation test was longer in arsenic exposure groups than that in the control group, the latent time was longer with the increase of arsenic dosage and the effect lasted until the fourth day of the test (P<0.05). In spatial probe test, the time for standing in the quadrant of original platform in arsenic exposure groups was shorter (P<0.05), the swimming distance to the quadrant of original platform and the frequency of crossing original platform was less in exposure groups (P<0.05). The expression of C-Jun protein was higher with the increase of arsenic dosage (P<0.05), whereas the level of C-Jun mRNA in high dose group was lower compared with the control group (P<0.05).Conclusions Sub-chronic arsenic exposure could induce an excessive expression of Jun protein, whereas the level of C-Jun mRNA was down-regulated and the learning and memory ability was reduced. The abnormity of C-Jun expression might be one of important intermediate links in the injury of learning and memory ability.