It has been found that rodents are suitable test animals (or use in determining the potency of senna, and in the present work mice were used. The assay procedure described involves administering the preparation under test at three dosage levels selected with a view to producing positive laxative effects in approximately 25, 50, and 75 of three similar groups of mice. Concurrently, a suspension of a standard senna powder was given at three appropriate dosage levels. The number of mice on each dose of unknown and standard showing laxative effects during the five and a half hours after dosing were recorded and from these data the potency and confidence limits were calculated by conventional methods. The method is applicable to senna in its various pharmaceutical forms. Satisfactory dosage-response relationships were found in experiments on humans. The potency ratios found on human subjects agree remarkably well with those on mice. This finding appears to validate the biological assay on mice and to facilitate stability studies on senna and its preparations.