Castor oil and magnesium sulfate were studied for their effects on gastrointestinal contractile activity in vivo. Ricinoleic acid, the active ingredient in castor oil, magnesium sulfate and mannitol were studied and compared for their effects on net water and electrolyte absorption in vitro. Extraluminal strain gauge transducers were implanted in dogs and used to monitor the circular smooth muscle activity of the antrum, duodenum, ileum and colon after water, castor oil or a 30 percent solution of magnesium sulfate. Substances were tested during the interdigestive (fasted) and digestive (fed) states. Decreases in total activity were found for both cathartics in the antrum and ileum. Further analysis revealed that decreases in the ileum occurred primarily by a decrease in contractile rate, whereas antral decreases could be attributed primarily to a decrease in force per contraction. Proximal colonic activity tended to decrease after laxatives and feeding. The effects of ricinoleic acid, isotonic and hypertonic solutions of magnesium sulfate and mannitol on net water and electrolyte absorption were tested on everted segments of hamster jejunum. Sodium ricinoleate (2.0 mM) reduced net water transport by 48 percent (P smaller than .01). Magnesium sulfate, like mannitol, only reduced net water absorption when present as a component of a hypertonic mucosal solution. The results suggest that both inhibition of water absorption and reduced circular smooth muscle activity may be important factors in castor oil- and magnesium sulfate-induced catharsis.