To study the intestinal absorption ofpyridoxine (PN), pyridoxal (PL) and pyridoxal 5'-phosphate (PLP) in rats, each B_6 compound was administered to the lumen of the ileal segment in situ and the amounts absorbed in the mesenteric venous blood collected from the segment were measured by high-performance liquid chromatography. PN, PL and PLP showed different absorption behaviors, e.g. PN and PL being absorbed more rapidly. PN was absorbed at a constant rate, which increased as the amount of PN administered to the lumen was increased. The PL absorption rate reached maximum 20 min after luminal administration and then decreased rapidly. The rapid decrease in PL absorption was not due to the decrease in the ability of the intestinal mucosa to absorb PL. PLP was absorbed intact, namely as the phosphorylated type at a slower rate than PN and PL. The absorption rate increased up to 80 min and there after remained constant. When PLP (5×10^<-3> M) was administered to the lumen, its concentration in the plasma of mesenteric venous blood reached 5.410^<-6> M. PLP absorption increased remarkably with venous infusion of noradrenaline (0.5 /μg/min) for 40 min and its concentration in plasma reached 4.610^<-5> M.