The role of regulatory peptides is the least explored part in the field of humoral regulation of lymphatics. In this paper we continue a systematic investigation of their effects on the lymphatic vessels of various animals. The effect of thyroliberin was studied on bovine mesenteric lymphatics. The isolated lymphatics contractility and smooth muscle cell electrical activity were investigated. Thyroliberin in ultra low concentrations (1 x 10(-13)-1 x 10(-18) M) exerts a considerable stimulating effect. The mechanism of the theroliberin ultra-low concentrations action and a possibility of the medical usage of the obtained results, are discussed.