Objective To determine the physiological role of ANH, which is thought to mediate the activity of the renin-angiotensin-aldosterone system and thereby control vascular tone and blood pressure. Despite more than ten years of research to determine the role of ANH in the regulation of aldosterone secretion, kidney function, and vascular tone and blood pressure, the findings remain controversial. This is largely due to the lack of availability of specific antagonists of ANH for the study of its physiology. Two antagonists (one is a peptide and the other a polysaccharide) have recently become available and we conducted some pilot studies to evaluate their effectiveness and to study ANH physiology. Adult male rhesus monkeys were anesthetized and fitted with two venous catheters, one for infusion and the other for sampling. Each monkey was studied four times with 2-3 weeks separating each study. All studies began with a period of saline infusion while the monkey equilibrated to the supine position and then vehicle infusion. Blood pressure and heart rate were recorded every five minutes and blood samples were collected at 30-minute intervals for measurement of plasma aldosterone, plasma renin activity and ANH levels. During the experimental phase there was (i) continued infusion of vehicle, or (ii) infusion of ANH plus antagonist, or (iii) infusion of antagonist only, or (iv) infusion of ANH only. Results included the expected effects of ANH and antagonist when given separately, but a paradoxical effect of the combined administration; antagonist effect prevailed in this condition. Further studies were conducted to assess the possibility that the paradoxical effect was due to an interaction of ANH and antagonist in the infusate prior to administration or to an imbalance in selected dosages of the substances. An application for addition funding to study ANH physiology in rhesus monkeys was prepared and submitted for a VA Merit Award to Dr. Shenker. The physiology of ANH was also studied in vitro using macaque kidneys procured through the WRPRC Tissue Distribution Program. Key words water and electrolyte regulation, blood pressure regulation, kidney function