Serial determinations of serum renin activity (RA) have been made in 19 patients recipient of 21 allografts. Five patients received kidneys from 4 related and from 1 unrelated living donors; 13 patients received allografts from 15 adult cadavers, and 1 from an anencephalic newborn. During the first post transplant month observations were made in 18 recipients. RA was found increased transiently in 7 and persistently in 10 recipients. The increase in RA was associated with acute tubular necrosis, 5; occlusive vascular lesions of the graft artery or its branches and infarction of the graft, 4; cortical necrosis, 2; and acute rejection, 3. In 2 recipients no apparent cause for the rise in RA could be found, and in 1 the increase was probably related to diuretic therapy. During the late post transplant period, from 2 months to 2 years and 8 months, observations were made in 10 recipients. RA remained increased in 2 recipients with occlusive vascular lesions and rejection of the allograft, respectively. In 6 other recipients RA increased in association with rejection, 4; development of graft artery stenosis, 1; and diuretic therapy, 1. In 2 recipients with uneventful course RA remained normal. There was no statistically significant correlation between changes in RA and endogenous creatinine clearance, or body weight. However, there was a significant negative correlation between RA and sodium excretion, and a small positive correlation between RA and diastolic blood pressure.