Abstract 1. 1. Plasma levels of l-aspartate: 2-oxoglutarate aminotransferase (AST) were estimated in Hereford cattle. 1 month to 12 years of age. kept under range conditions and in a group of Hereford × Angus cows kept on the same range. 2. 2. Plasma levels of AST were estimated in a group of Crossbred cows and their calves fed a constant diet and kept in individual pens in the same geographic area as the Herefords. 3. 3. Seasonal changes in mean plasma AST were observed in the Herefords corresponding with the change from dry winter grasses/hay and well water to fresh spring and summer grasses and slough water. No seasonal changes were observed in the Crossbreds given a constant dry diet and city water. 4. 4. Plasma AST increased with age in calves 1 to 12 months of age in the Herefords but not in the Crossbreds. Mean plasma AST did not change with age in any of the adult cattle studied. 5. 5. Small increases in plasma AST corresponding to increases in ambient temperature above −12°C were observed in the Crossbreds. 6. 6. An increase in plasma AST was observed near the time of first ovulation in the confined cows. 7. 7. No relationship could be demonstrated between plasma AST and sex, breed or time to parturition in the range cattle. Breed differences were observed in the Crossbred cows.