Following the course of the dermatitis caused with croton oil in the back of rabbits, sodium, potassium and calcium in sera and skin were determined spectrophotometrically. 1. The quantity of sodium in sera showed a slight decrease in the initial stage of the dermatitis, showed increase in the severe stage and moreover increased in tho retrogressive stage and returned to the normal value in the recovering stage. In skin, increasing from the initial stage, showed marked increase in the severe stage and then decreased rapidly and showed nearly normal value in its retrogressive stage. 2. The quantity of potassium in sera increased slightly and showed remarkable increase in the severe stage, but, decreasing rapidly, showed almost normal value in retrogressive stage. The variation of potassium in skin showed the same tendency as sodium but, in the initial stage, increase of potassium was more remarkable than sodium. 3. Calcium in sera showed aslight increase in the initial stage, showed the lowest value in the rotrogressive stage and then gradually increased but returning to the normal value was delayed. In skin, showed a slight increase in the initial stage, showed the lowest value in the severe stage but thereafter was elevated gradlualy. 4. The value of Na/K in sera showed the lowest in the severe stage and in skin the lowest was showed in the initial stage and the both declined early and were eleavtated later. 5. K/Ca in sera showed the highest value in the retrogressive stage and, in skin, the highest was showed in the severe stage and the both were elevated in early period and dropped in the latter half. 6. Almost similar tendency of the variation of electrolytes was showed in the sera and the skin, but in latter more remarkable.