The action of three different kinds of progesterone related compounds was tested on the spontaneous contractility of the isolated guinea pig ileum. The results showed that this tissue was very sensitive to the action of androgens, progestins and corticosteroids. Relaxation was observed as dose-dependent; however, effective doses 50 were different, being related to the chemical structure of the compound. 5 beta-progestins were the most potent steroids followed by 5-reduced androgens (5 alpha or 5 beta). 4-en compounds such as testosterone, progesterone, 17 alpha-OH-progesterone, and corticosteroids were the lower in potency. 5 alpha-pregnanedione and pregnanediols were practically ineffective. The high sensitiveness of the ileum smooth muscle makes this organ a target for steroids. Thus, steroidal influence on the intestine might be related with contractility changes in some physiological circumstances, e.g., pregnancy.