The previous 2 reports of the Veterans Administration Cooperative Study on Antihypertensive Agents described the results obtained with reserpine, hydralazine, and 3 ganglion blocking drugs.1,2 The present investigation is concerned with the evaluation of chlorothiazide both alone and in combination with the agents listed above and with cryptenamine, an alkaloid derived from Veratrum viride. Although these latter studies are still in progress, the results seemed of sufficient interest to warrant a preliminary report at this time. Methods This investigation used 2 separate protocols for each of 2 different groups of patients. The first (Group A) were untreated cases entering the study for the first time. The second (Group B) were patients who had been under treatment in the cooperative study for at least 1 year,1,2 after which chlorothiazide was added to the regimens of the patients taking active medications. The criteria used for selecting patients and for classifying