Fresh milk, boiled or umboiled, does not have any action on the tonus of a preparation of an isolated and perfused rabbit's gut, however it produces a considerable increase in the height of the rhythmic contractions. Soured milk, naturally fermented or by addition of colon or aerogenes bacilli from 6 to 24 hours at 37° C., may evoke a considerable increase in the tonus of the preparation. Both alterations are, however, not so intense as those obtained with filtrates of old cultures of colon bacilli, as refered to in previous papers. The action on the tonus does not appear to depend onthe acidity of the soured milk because neutralised soured milk does not prevent the phenomenon to be present. On the other hand a lactic acid solution inthe same concentration as the soured milk - expressed by Dornic degrees - does not increase the tonus of the preparation. Heating up to 115° C. or prolonged cooling to + 3° C. does not prevent also the activity of the naturally or artificially soured milk onthe tonus as we have also described with the filtrates of old cultures of colon bacilli. By previous propination to rabbits of filtrates of colon bacilli, or artificially fermented milk by the same bacteria, a great increase of intestinal reactions was noticed, as compared with experiments performed upon the intestine of normal rabbits. Identical results were also obtained with filtrates of cultures of colon bacilli, as referred to previously.