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The genetically hypertensive fawn-hooded (FHH/Eur) rat is characterized by the early presence of systolic and glomerular hypertension, progressive proteinuria (UPV), and albuminuria (UAV), and focal glomerulosclerosis, resulting in premature death from renal failure. Previous studies showed that at least five genetic loci (Rf-1 to Rf-5) were linked to the development of renal impairment. Of these five, Rf-1 appears to play a major role. To study the impact of Rf-1 in the absence of the other loci, we transferred the Rf-1 region of chromosome 1, between the markers D1Mit34 and D1Rat156, Rf-1B for short, onto the genomic background of the normotensive August x Copenhagen Irish (ACI) rat. In this congenic strain, named ACI.FHH-D1Mit34/Rat156 or ACI.FHH-Rf1B, we challenged the renal hemodynamic function of these animals by studying the effects of unilateral nephrectomy (UNX) alone, or combined with N(G)-nitro-l-arginine methyl ester (l-NAME)-induced hypertension. Following UNX, the congenic strain developed significantly more UPV and UAV than the ACI progenitor. The differences were even more pronounced when UNX was combined with an l-NAME-induced rise in systolic blood pressure to about 150 mmHg, i.e., the level of hypertension present in the parental FHH strain. These findings indicate that the Rf-1B region of the FHH rat contains at least one gene affecting the susceptibility to progressive renal failure, especially in the presence of an increase in blood pressure.