1. Plasma levels of lactate dehydrogenase (LD) were estimated in Hereford cattle of 1 month to 12 yr of age kept under range conditions. 2. Plasma levels of LD were estimated in a group of confined cows and their calves fed a constant diet and kept in individual pens in the same geographic area as the range Herefords. 3. Large seasonal changes in mean plasma LD were observed in the range cattle but not in the confined, constant diet group. The seasonal changes corresponded with the appearance of new growth on the range. 4. Changes in plasma LD were observed at weaning time in the range cattle but not in the confined group. 5. Sex had no effect upon plasma LD levels in the juvenile animals of either group. No comparison of adult animals could be made. 6. Mean plasma LD levels did not change with age but the seasonal effect decreased in magnitude as the animals matured. 7. An increase in plasma LD was observed around the first ovulation in the confined animals. 8. A decrease in plasma LD was observed during parturition in the confined group.