The experimental regeneration of patella was studied in 15 young mice. After ex-cision of the patella these animals were raised under different living conditions. Ingroup A, the animals were kept under daily training for an hour by running on a turningwheel in order to intensify the movement at the knee joint. In group B, the animalswere kept in a cage large enough for free movement. In group C, the animals were con-fined in an apartment in the cage where their movement was limited to minimum. Afterthree months, the regeneration of the patella in these animals was examined by means ofroentgenographic and histologic methods. The results were as follows: After the patella was excised in young mice, a new one was formed if free movementat the knee joint was allowed. If, on the other hand, the movement was limited arti-ficially, no regeneration of the patella occurred after three months. But if the movementwas intensified by training, the formation of a new patella was accelaerated. Therefore,it may be held that the formation of the new patella is dependent upon the conditionof the functional activity of the knee joint. In addition to the new petella, some ac-cessory ones were also formed in the running group. The above results indicated that such mechanical factors as compression force, tensileforce and frictions at the tendon of the quadriceps muscle are indispensible conditions forthe regeneration of the new patella. The histological evidence also shows that the formation of the new bone was broughtabout through the fibrochondral ossification, and no intramembraneous ossification wasobserved. The structure of the new patella was characterized by all the morphologicalfeatures commonly seen in the cancellous bone. The time required for the regenerationof the new patella in mice ranged from 1.5 to 2 months. The mechanisms in, and condi-tions for the formation of new patella were dicussed from the aspect of functional mor-phology.