The objective of this study was to examine the influence of the commonly used antidiarrheal agent loperamide on the internal anal sphincter of anesthetized opossums. Loperamide caused a rise in internal anal sphincter pressures, and a decrease in the rectal distention-induced internal anal sphincter relaxation. To define the mechanism of decrease in the rectal distention response of the internal anal sphincter, we examined the effects of sacral efferent nerve stimulation, local intramural stimulation, and ganglionic stimulant (which acts at the postganglionic inhibitory neuron). Loperamide caused a significant decrease in the internal anal sphincter relaxation caused by sacral nerve stimulation, but not in that caused by local intramural stimulation and nicotine. These effects of loperamide on the internal anal sphincter were antagonized by naloxone. These findings suggest that the therapeutic effects of loperamide on the internal anal sphincter as an antidiarrheal agent are due to a rise in the tone of the sphincter and the inhibition of its relaxation in response to rectal distention. Both of these actions are mediated by activation of opioid receptors. The inhibition of internal anal sphincter relaxation induced by rectal distention (which mimicks recto-anal reflex) is due to inhibition of release of neurotransmitter from the preganglionic sacral nerve fibers.