Summary A marked increase in the concentration of total blood and urinary acetone bodies and their component fractions of acetone and acetoacetic acid, and β-hydroxybutyric acid was observed during the late winter and early spring months with animals receiving molasses-treated grass silage. The lowest concentrations of these substances were observed in these animals when they were on early spring and summer pastures and were receiving grain, hay, and only a relatively small quantity of silage. It is believed that the relatively high values obtained for the group as a whole during the winter feeding period were due primarily to grass silage feeding. Although acetone and acetoacetic acid, and β-hydroxybutyric acid increased, the latter fraction was responsible for most of the rise. Since the feeding of fresh silage did not produce this effect, it appears that the increase in the acetone bodies on silage feeding was associated with the aging of the silage. In the light of the work of Bender et al . (2) and MacKay et al. (12) it is suggested that the increased production of the acetone bodies due to the feeding of aged molasses-treated grass silage is due primarily to the volatile acids in the silage. Frequent observations on the blood and urinary acetone bodies of six cows showed a sharp decline in these substances immediately after the animals were placed on spring pasture. The blood and urinary acetone bodies had decreased 52.8 per cent and 51.5 per cent respectively after 16 days of daily pasture feeding. The maximum decreases in acetone and actoacetic acid were observed following the first day of pasture feeding. Blood and urinary (β-hydroxybutyric acid, however, continued to decline throughout the 16-day period of observation. The sharp decline in acetone bodies after one day of pasture feeding was undoubtedly due primarily to the decreased silage feeding. The pasture grass feeding is believed to be partially responsible for the further continuous decline in the acetone bodies. The blood acetone body levels of four heifers which were pasture fed during the summer months and received only mixed grass hay of a fair quality during the winter months were much lower while on hay feeding than on pasture feeding. The blood acetone bodies increased following feeding, reaching a peak three hours after feeding, and then gradually decreased, returning to the pre-feeding level within nine hours. This effect was much more evident when aged grass silage was included in the ration.