SUMMARY: Mucosal fluid, sodium and chloride transfer were measured in everted sacs of rat, guinea-pig and hamster jejunum, and in rat ileum and colon, and in guinea-pig gall bladder. After treatment of the animal with ovine prolactin, a highly significant enhancement of fluid and NaCl absorption was observed in rat, hamster and guinea-pig jejunum. Prolactin treatment caused a significant increase in fluid and NaCl transfer in rat ileum, but not in guinea-pig ileum or rat colon. Prolactin administration had no consistent effect on fluid and NaCl absorption by the guinea-pig gall bladder. The several regions of the mammalian gut appear to differ in their responsiveness to prolactin.