This study was a parallel, double-blind, placebo-controlled evaluation of active doses of nifalatide, an enkephalin analog, in subjects with castor oil-induced diarrhea. Seventy-two subjects completed the study. The time to first stool after castor oil administration was significantly greater than after the 16 and 48 mg doses of nifalatide as compared with placebo dosing. The same doses of nifalatide also decreased the overall stool frequency, the frequency of abdominal cramping, and the incidence of nausea and vomiting. There were no clinically significant, drug-related changes in the physical examination results, ECG, vital signs, or clinical laboratory parameters. The only increased adverse experiences that appeared to be related to the drug were dizziness and mild dry mouth.