The effect of infusion of hyperonciotic (25 g% salt-poor bovine plasma albumin) (0.06 g% of body weight) has been studied using clearance and micropuncture techniques in animals with two forms of immunologically induced experimental glomerulonephritis and in age-matched control rats. In control rats the significant natriuresis and diuresis after hyperoncotic albumin were associated with decreased fractional and absolute proximal reabsorption, decreased calculated efferent oncotic pressure, and a transitory rise in efferent arteriolar hydrostatic pressure. These findings, however, do not necessarily indicate a predominant role for peritubular "physical factors" in the control of proximal reabsorption. In glomerulonephritic rats a smaller diuresis and an insignificant natriuresis were found after hyperoncotic albumin, especially in anti-GBM nephritis, when no change in SNGFR or proximal absolute reabsorption occurred. In AICN there was a rise in SNGFR and a fall in absolute reabsorptive rate, especially in those nephrons with high filtration rates. There was no evidence that any alteration in efferent arteriolar hydrostatic pressure of calculated efferent oncotic pressure had occurred.