Two cases of acute renal insufficiency occurred in association with episodes of severe encephalitis due to herpes simplex type I. The possibility was considered that the renal failure was due to viral infection of the kidneys, and animal experiments were carried out in an attempt to confirm this. Young New Zealand albino rabbits were infected i. v. with HSV type I; the virus antigen was detected in the kidney of 8 of 10 animals, and IgG was found on the GBM in 9 of 19 animals. Viruria was observed in 12 of the 29 infected animals, and electronmicroscopic examination confirmed the presence of immune complexes in the glomeruli.