Mouse epidermal keratinocytes (MEK) cease growth in vitro after a finite number of population doublings (PD). To investigate the effect of low oxygen tension (LOT) on the in vitro life span of mouse epidermal keratinocytes, MEK were cultured under four different oxygen conditions (1%, 3%, 10%, 20% O-2). The results showed that a longer replicate life span was achieved under the reduced oxygen conditions. When the primary keratinocytes dissociated from mouse epidermis were cultured and serially subcultured under 1% O-2 the in vitro life span increased by 49.0%. When the cells cultured under low oxygen tension were transferred to conventional atmospheric condition (20% O-2), they senesced faster than the counterparts under low oxygen conditions, but they still had an extended life span compared with the cells cultured under 20% O-2. It was found that when the culture condition (20%, O-2) for cells at the terminal senescent stage was shifted to 3% O-2, no prolonged life span was observed.