Based on the EEG-observation of falling asleep when patients are under peloid therapy applied on the right hypochondrium, following assumption is made to clarify the mechanism of inducing sleep. While patients are under the peloid therapy, some chemical substance will be produced in circulating blood; L-tryptophan, an essential amino acid and a precursor of serotonin, gets over a blood-brain barrier easily to induce sleep. Six healthy volunteers, each sex by threes, were selected to apply peloid pack on the right hypochondrium for 30 minutes following 12-hour fasting. Significant increase in plasma L-tryptophan level was obtained in the clinical experiment. It is speculated from the preliminary data that L-tryptophan increased by the peloid therapy may possibly act as a specific physiological hypnotics.