Partial thickness articular cartilage injuries (PTCIs) were not previously thought to heal spontaneously. Immature rats have the capacity for spontaneous repair of PTCIs, although it is a long-term process. Our aim has been to examine the spontaneous repair response mechanism in immature rats. Single linear PTCIs were created in 3-week-old and 12-week-old rats in the direction of joint motion. On day 1 and at 1, 2, and 4 weeks after PTCI, evaluations of histological changes and immunohistology at the injury site and in the surrounding cartilage were performed. Anti-CD105 and anti-CD166 antibodies (as stem cell markers to identify mesenchymal stem cells in reparative cartilage tissue) were used for immunohistological evaluations. To determine whether endogenous repair ability existed in articular cartilage, an ex vivo experiment was also carried out. Femoral condyles with PTCIs were incubated in Dulbecco's modified Eagle's medium containing 10% fetal bovine serum for 1 day and for 1 and 2 weeks. Histological changes were subsequently examined. Immature cartilage showed a higher repair response than did mature cartilage, and the response occurred immediately after PTCI. In immature rats, CD105- and CD166-positive cells were found in the superficial and transitional zones of the articular cartilage. Few CD166-positive cells were identified in mature articular cartilage. No significant in vivo differences in the spontaneous repair responses to PTCIs were observed between mature and immature groups. Thus, the repair response to PTCIs seems to be associated not only with CD105- and CD166-positive cells, but also with other perichondral factors.