ABSTRACT In a previous paper the relative rates of absorption of acetic acid and propionic acid through the rumen wall were discussed (Gray, 1947a). While the manuscript was being prepared, Danielli, Hitchcock, Marshall & Phillipson (1945) described a series of experiments from which it was evident that the lower homologues of the fatty acids are absorbed through the rumen wall at different rates when the rumen contents are acid, and that the relative velocities of absorption fall in the series butyric > propionic > acetic. The observations made in this laboratory (Gray, 1947 a) independently and by different methods were in agreement with these conclusions. There is therefore no reason to doubt that the higher homologues are absorbed preferentially when the rumen contents are acid.