Drug interactions with antihypertensive drugs can be either beneficial or hazardous. The hazardous interactions are relatively infrequent but must be shown so they can be avoided. Those of most importance involve interaction with guanethidine-type agents and tricyclic antidepressants, amphetamine-type anorexiants or phenolpropanolamine-type common cold remedies; combined use of potassium retaining diuretics with potassium supplements; and incautious use of diuretics with cardiac glycosides. The beneficial interactions are the basis for modern antihypertensive therapy and can be of major help if logically applied to therapeutic problems.