The intestinal absorption of PABA, a member of the vitamin B complex, was characterized in vivo and in vitro in the rat by the segmental intestinal perfusion and everted gut sac technique. Net PABA absorption was directly proportional to substrate concentration, and saturation of absorption did not occur with increasing concentrations of PABA (1 to 50 mM), indicating a nonsaturable process. Jejunal and ileal absorption rates were similar and were not influenced by the presence of glucose or the absence of sodium in the test solution. Similarly, 14C-PABA transport in vitro was nonsaturable and proportional to PABA concentration (0.05, 0.5, 1, 10, and 50 mM). It was not inhibited by ouabain or other PABA analogues such as folic acid and benzoic acid. These studies indicate that PABA, a vitamin B cofactor, is absorbed by a nonsaturable, sodium-independent process, which characterizes passive diffusion and is similar to the absorption of other vitamin B members.