Eccrine sweat secretion is studied during physical stress after the administration of a B-blocking agent: Butofilolol. 20 voluntary normotensive subjects, age 24 + 2 ans are submitted to 2 exercises stress test (ergometric bicycle) performed at on week interval. Butofilolol is administrated at a posology of 200 mg, 24 hours and 2 hours, before the test as well as placebo (double blind). Local sweat output is measured 1 min after stress and in the interscapulary region with an evaporimeter. Sweat weight, sweat electrolyte concentrations (sodium, chloride) are analysed from filters papers placed on the forehead. With butofilolol, significative increases of sweet output (+13%), sweat weight (+20%), and sodium (+20%), chloride (+26%) elimination are observed. The subjective observation of the effects of B-blocking agents is confirmed. The way of action of Butofilolol discussed.