One half-udder from a lactating cow was perfused for 110 min with blood containing 1 mc of [1-14C]butyrate. Inactive sodium butyrate was added at the eightieth minute. The volume of milk collected was 320 ml. One half-udder from another cow was perfused for 120 min in the presence of 0·5 mc [3-14C]butyrate, inactive sodium acetate being added at the sixtieth minute. The volume of milk collected was 150 ml.
The total amount of 14CO2 recovered was 5·7% of the [1-14C]butyrate given and 7·6% of the [3-14C]butyrate given. In both instances casein, followed by the volatile fatty acids, showed the highest specific activity of the constituents isolated from the milk. The glyceride fatty acids in the udder were fifty times as active as those in the milk. With [3-14C]butyrate the activity of the total fatty acids amounted to 24% and that of total lactose to 0·38% of the added 14C. Butyrate did not appear to be used for glycogenesis in the perfused gland. The specific activity of the lower fatty acids of both the udder and the milk increased stepwise with increasing chain length to reach a maximum at C10. A satisfactory explanation for this peculiar 14C distribution cannot be given at the present time. There was no evidence of direct esterification of butyrate. Most of the activity of casein was due to labelled glutamic and aspartic acids, the activity of the former being four times as high as that of the latter. The acids of the Krebs cycle isolated from the udder tissue when [3-14C]butyrate was given showed very high activity. No striking differences were observed between the results of the two experiments. It is concluded that butyrate is split into two C2 components which behave identically. These are utilized for fatty acid synthesis and take part in the Krebs cycle. The relative 14C distribution between the components isolated from milk and those from tissue may be a reflexion of the secretory processes in the udder cells, synthesized fat tending to be secreted in the alveoli after the other constituents.