Transport across the wall of rat jejunum of two isomers of methyl red has been studied. These isomers, meta-methyl red (MMR) and para-methyl red (PMR), are absorbed against a concentration gradient. Uptake consists of three components: rapid adsorption on to the mucosa, a steady uptake proportional to lumen concentration and a reflux which increases exponentially to a limiting value. A substantial part of the uptake is stored in the mucosa, and some of it is metabolized to colourless derivatives. If methyl red is washed out of the lumen after the mucosa has been loaded, there is some reflux into the lumen, but most of the stored methyl red passes into the secretion. The rate of this transport on to the serosal surface is not noticeably diminished until the adverse concentration gradient exceeds 13-14:1. Once the methyl red has been washed out of the lumen there is no further metabolism of methyl red to colourless derivatives. Shifting the lumen pH from 7 4 to 6-4 whilst keeping the tissue fluid pH constant increases the rate of uptake from the lumen by a factor of only 1 68 although the concentration of unionized methyl red is increased approximately 10-fold. It seems that at pH 7 4 about 900% of the uptake is of the ionized form, and that at pH 6 4 this percentage falls to 50.It is concluded that tl-methyl reds form fresh examples of foreign organic compounds which can be transp' . ted actively by the small intestine and that they can be taken up from the lumen into the mucosa in both ionized and unionized states.Schanker, Tocco, Brodie and Hogben [1958] and Hogben, Tocco, Brodie and Schanker [1959] studied the intestinal absorption of foreign organic compounds from perfusates passing very slowly through rat small intestine in vivo and concluded that the absorption observed was due to diffusion out of the lumen of uncharged lipophilic molecules. Their sole direct evidence for diffusion was that both salicyclic acid and aniline were absorbed at rates proportional to lumen concentration over ten-fold ranges of concentration. This work has led to the generalization that all foreign organic compounds are absorbed by passive diffusion across the intestinal epithelium except where they can make use of an active system transporting a natural substance [Schanker, 1971]. However, as pointed out by Fisher and Gardner [1974], absorption from slowly moving perfusate is limited by the need for the absorbable solute to diffuse radially through the lumen contents to the mucosa. Since this radial diffusion is rate-117