Summary Serum from lambs with nutritional muscular dystrophy showed increases in all 5 forms of lactic dehydrogenase (LDH). The largest percentage increase occurred in LDH V, followed by isoenzymes IV, III, II and I. The largest absolute increase was in LDH III. Therapy with vit. E alone or with vit. E plus selenium restored normal levels of all isoenzymes. Treatment with selenium alone diminished serum LDH only temporarily. Data are presented for the LDH isoenzymes in lamb tissues. Skeletal muscle from dystrophic lambs showed a decrease in the percentage of LDH V and increases in the percentages of the other LDH forms. The other tissues from dystrophic animals showed an essentially normal LDH isoenzyme distribution.