The two sclerosants widely used in the treatment of varicose veins of the leg are tetradecyl sulphate of sodium and ethanolamine oleate. Tetradecyl sulphate of sodium is also used in the treatment of haemorrhoids and in this situation its competitor is five percent phenol in lamond oil. Hotherto no comparison of the effects of these three sclerosants appears to have been published, despite their very extensive use. The present work, in rats, reports on their efficiency as vein sclerosants, the mortality following intravenous injection and the incidence of ulceration following intradermal or subcutaneous injection. A detailed study of the effect on the veins of injection of tetradecyl sulphate of sodium is reported elsewhere.