This study examined the emergence of delayed behavioral effects in offspring mice exposed to low levels of mercury vapor (Hg⁰) during the lactation period. Female offspring of mice were repeatedly exposed to Hg⁰ at 0.057 mg/m³, similar to the current threshold value (TLV), for 24 hr until the 20^th day postpartum. The behavioral effects were evaluated with locomotor activity in the open field (OPF), learning activity in the passive avoidance response (PA) and spatial learning ability in the Morris water maze (MM) at the ages of 3 and 15 months. Hg⁰-exposed mice did not differ from controls in the three behavioral measurements at 3 months of age, and no neurobehavioral effects were observed. On the other hand, the mice exhibited significantly more central locomotion in the OPF task when tested at 15 months of age, but no abnormality in other behavioral performance. Immediately after postnatal exposure, the brain mercury concentration of offspring was about 150 times that of the control, in which the concentrations were approximately 0.4 μg/g. The results indicate that mice exposed to Hg⁰ at concentrations around TLV during the developing period resulted in the emergence of delayed behavioral effects at a later stage in life.