A study has been made on the skin response of mouse hind limb to radiation to evaluate the difference of skin response to superfractionation and conventional fractionation schedules, and to optimize the time interval between and the dose per fraction in the superfractionated irradiation. 96 mouse hind limbs were devided into 12 groups and were irradiated with 10 consecutive fractions by intervals of 6, 12 or 24 hours and dose per fractions of 400, 500, 600 or 700 rads. The skin changes of the irradiated hind limb were observed for 30 days and the skin response were analyzed. The results are as follows: 1. There was no significant difference of early skin response along the time interval, from 6 to 24 hours, up to 600 rads per fraction. 2. Mean duration to maximum skin reaction in superfractionation (15.48$pm$2.80 days) is shorter than in conventional fractionation (18.05$pm$3.20 days) by 2.57 days. (P<0.05). 3. Optimum time interval between fractions in superfractionation may be 6 hours or less. 4. Optimum dose per fraction in superfractionation may be 500 rads or less,