PRELIMINARY observations,1reported three years ago, indicated that hydrallazine (1-hydrazinophthalazine; apresoline®)2possessed certain desirable features as a therapeutic agent for the treatment of hypertension. It reduced the blood pressure, increased renal blood flow,3and was effective following oral administration. Since the original report we have extended our studies and now have available a longer follow-up evaluation (1 to 2 years). In addition, some pharmacological data have been collected concerning the mode of action of this compound. As the blood pressure is reduced, the heart is stimulated and cardiac output increases.4The latter response is the cause of some of the most undesirable side reactions and is probably a result of central stimulation of certain components of the sympathetic nervous system, since it is abolished by sympathectomy5and ganglionic blockade.6The purpose of the present communication is to review some of the more important aspects of the pharmacology of this drug