The precise mechanisms involved in the association of hemoglobinemia with acute renal failure remain unknown. The difficulties in studying this problem in patients are manifold, consequently the bulk of the investigation in this area has necessarily involved experimentally produced lesions in the laboratory animal. Largely as a result of such studies a variety of differing and conflicting hypotheses concerning the pathogenesis of this renal lesion have arisen. Perhaps the most favored of these are the suggestions that (a) tubular blockage by precipitated hemoglobin is the primary cause of excretory failure in the affected kidney, and (b) some undefined factor or factors produce a primary reduction in glomerular filtration rate, presumably as a consequence of renal ischemia. Carefully performed studies may be cited in support of either the first (1, 2) or second (3, 4) of these hypotheses. In addition, other explanations for the renal failure have been considered, particularly the role of abnormal back diffusion of tubular fluid through damaged tubular epithelial structures and the effect of renal edema in reducing renal blood flow and filtration rate. The establishment of an experimental model of a clinical renal disease in the laboratory animal affords the opportunity of making accurate functional measurements of the evolution of the lesion and of correlating structural alterations at each stage of the disorder with the corresponding functional data. This does not appear to have been accomplished with acute renal failure associated with hemoglobinemia. This lack of serial functional and morphological studies of a standard lesion may in part account for the conflicting views regarding the pathogenesis of this disorder. Consequently the present study was designed, with the ultimate objective of defining the mechanisms underlying the renal functional failure associated with this lesion.The present report describes a relatively simple method for the production of acute renal failure in the rat by the intravenous injection of hemoglobin. The lesion is reversible and relatively uniform in its functional and structural manifestations. Certain of the factors which influence its production and degree of severity have been examined. The functional evolution of the lesion, from its