Keap1 regulates Nrf2 activity in response to xenobiotic and oxidative stresses. Nrf2 is an essential regulator of cytoprotective genes. Keap1-null mice are lethal by weaning age due to malnutrition caused by severe hyperkeratosis of the upper digestive tract. Analysis of Keap1::Nrf2 double mutant mice revealed that currently recognizable phenotypes of Keap1-null mice are all attributable to constitutive activation of Nrf2. We previously reported that hepatocyte-specific Keap1 knockout (Keap1(flox/-)::Albumin-Cre) mice are viable and more resistant to acute toxicity of acetaminophen (APAP). In the current study, we found that the floxed Keap1 allele is hypomorphic and that Keap1 expression was decreased in all examined tissues of Keap1(flox/-) mice. Taking advantage of the hypomorphic phenotype of Keap1(flox/-) mice, we examined the effects of graded reduction of Keap1 expression in adult mice. When challenged with APAP, Keap1(flox/-) mice were more protected from mortality than wild-type and even Keap1(flox/-)::Albumin-Cre mice. In contrast, a decrease in Keap1 levels to less than 50% resulted in increased mortality in a study of 2-year-old mice. These results support our contention that the benefits of Nrf2 activation in acute toxicity are hormetic and that constitutive Nrf2 activation beyond a certain threshold is rather disadvantageous to long-term survival.