The present study was designed to examine the effects of bilateral adrenalectomy and subsequent administration of dexamethasone or aldosterone on plasma angiotensinogen concentration (PAC) in the dog. A second purpose of the study was to determine if the changes in PAC which occur during these maneuvers influence the formation of angiotensin I. This was done by comparing the accompanying changes in plasma renin activity (PRA) and plasma renin concentration (PRC). Following adrenalectomy PAC decreased; PRA and PRC both increased but the increase in PRA was considerably less than the increase in PRC. Dexamethasone administration was associated with a marked increase in PAC; PRA also increased but PRC did not change significantly. On the other hand, PAC decreased during aldosterone administration; PRA also decreased while PRC usually increased. Thus these studies confirm previous reports that adrenalectomy decreases PAC and indicate that this decrease is a consequence of loss of glucocorticoid rather than mineralocorticoid activity. The divergent changes in PRA and PRC which occurred during adrenal insufficiency and steroid treatment were apparently the result of the associated changes in PAC. The influence of PAC on the velocity of formation of angiotensin I was also studied in vitro. Velocity increased as the angiotensinogen concentration was increased over a wide range and it was found that at physiological PAC the velocity of the renin-angiotensinogen reaction for any given concentration of renin is approximately ½ maximum. It therefore appears that in the dog changes in PAC may importantly influence the rate of formation of angiotensin I. (Endocrinology93: 107, 1973)