In order to carry out a detailed inveeti zat.ion into the origin and fate of the acetone bodies in the normal ruminant, a method was devised for the estimation of individual acetone bodies and .isopropanol. A detailed study of the fasting ketosis of both cows and sheep, and the metabolism of the individual ketones and iso-propanol in these two species was carried out. The results obtained indicated that acetone, acetoacetic acid and B- hydroxybutyric acid originate in the tissues, but the site of origin of iso-propanol is most probably the rumen A very high degree of interchange was found to occur between acetone, acetoacetic acid and isopropanol the blood, tissues, and rumen, but B hydroxybutyric acid appeared to be the end-point of the conversions being metabolised by the tissues without the production of the other ketones or isopropanol. Conditions of stress, such as fasting, produce a ketosis in both sheep and cows, and though there appears to be a slight species difference in the proportions of the individual ketones formed, the response of the two species in late pregnancy is very similar, and differs from that produced at peak lactation. These results are discussed and compared with those obtained by other workers in various animal species.