In this study, rats recovering from glycerol-induced acute renal failure were found to be protected from mercury-induced nephropathy, and HgCl2 poisoning protected rats from developing myohemoglobinuric renal failure. In view of the widely disparate nature of the renal failure models used, refractoriness appears to relate to an altered sensitivity of the organism itself rather than reflecting resistance to a particular nephropathic challenge. Renal renin content of the rats at the time of rechallenge was normal or high, a finding which contrasts sharply with that of chronically saline-loaded animals which also are refractory to ARF but have a maximally suppressed renal renin content. Renal renin depletion is not essential to the prevention of acute renal failure in the rat.