Female C57BL mice were exposed to a low level of mercury vapor (Hg⁰, 0.096 mg/m³) and was given the solution containing 5-ppm methylmercury (MeHg) during the growth period to examine the influence on the neurobehavioral function after birth. Exposure period was 4 weeks at 3 to 7 weeks of age. At 10 weeks of age, three behavioral tests were conducted; open field (OPF) test, passive avoidance response (PA) test, and eight-arm radial maze (RM) test. To evaluate the influence of aging, additional behavioral tests were performed at 79 weeks of age. With respect to the results of the three behavioral tests conducted at 10 to 14 weeks of age, there were no significant differences between the Hg⁰/MeHg/Hg⁰+MeHg and control groups. Furthermore, there were also no significant differences between each exposure and control group on behavioral tests performed at 79 to 83 weeks of age after the completion of mercury exposure. The concentration of mercury in the brain after the completion of exposure was the highest in the Hg⁰+MeHg group, followed by the MeHg and Hg⁰ groups. The values in the Hg⁰+MeHg and MeHg groups were ≤ 3.0 μg/g. The value in the Hg⁰ group was ≤ 1.0 μg/g. There were no differences in the brain concentration of mercury after 1 year between the Hg⁰/MeHg and control groups. However, in the Hg⁰+MeHg group, it was significantly higher than in the control group, suggesting that the disappearance of mercury in the brain is delayed in comparison with the exposure to Hg⁰ or MeHg alone. These results showed that there was no influence of low-level Hg⁰+MeHg exposure during the growth period on neurobehavioral manifestation. However, the disappearance of mercury in the brain was delayed in comparison with the exposure to Hg⁰ or MeHg alone.