Kidney bean lectin, PHA, stimulated the hyperplastic and hypertrophic growth of rat small intestine. This growth was preceded by a rapid accumulation of polyamines in the small intestine. However, since the lectin had little effect on in situ polyamine biosynthesis, most of the polyamines must have been of extracellular origin. To investigate the source of polyamines, both the luminal uptake and basolateral transport of polyamines by the rat small intestine were measured in vivo. Luminal polyamine uptake was apparently by passive diffusion, non-saturable, linearly dependent on concentration and its extent was not stimulated by PHA. In contrast, 14C-polyamines injected intraperitoneally were probably taken up by a transport system(s). Moreover, basolateral polyamine transport was stimulated in a time-dependent manner when small intestinal growth was stimulated by PHA. However, in keeping with the finding of polyamine accumulation prior to demonstrable growth of the tissue, stimulation by PHA of the polyamine transport system also preceded small intestinal growth. Stimulation of polyamine transport by luminal factors is possibly a general mechanism involved in intestinal adaptation.