Hematopoietic stem cells (HSCs) are the ultimate source of all blood and immune cell lineages. Under homeostatic conditions, blood cell production is maintained with steady, but marginal HSC contribution. Therefore, HSCs are perceived as a quiescent reserve population for situations of acute stress. As inflammatory cytokine signalling alters the immuno-phenotype of HSC, their analysis during acute hematopoietic perturbation remains challenging and findings of stem cell activity during stress hematopoiesis vary greatly. To circumvent this problem, we employed two mouse models capable of recording HSC differentiation and proliferation in a native environment and show that the activity and output of HSCs is only marginally changed in response to physiological hematopoietic perturbations. While myeloablative chemo- or radiotherapy significantly stimulated HSC contribution by accelerating proliferation and differentiation, we did not observe alterations of HSC output into mature blood cells under conditions mimicking thrombocytopenia or microbial infections. Immunological training, in which HSCs have been proposed to store and integrate information on previous innate immune encounters, did not alter HSC proliferation and differentiation upon secondary stimulation of innate immunity. In sum, HSCs only marginally accelerated their output to immediate progeny in response to hematopoietic perturbation. Therefore, these cells do not represent a reserve population that directly participates in regeneration of mature blood cells. Instead, we postulate that the core function of adult HSCs may reside in slow, but continuous rejuvenation of extensively dividing progenitor populations and thus ensuring genomic integrity. Hematopoietic stem cells (HSCs) are the ultimate source of all blood and immune cell lineages. Under homeostatic conditions, blood cell production is maintained with steady, but marginal HSC contribution. Therefore, HSCs are perceived as a quiescent reserve population for situations of acute stress. As inflammatory cytokine signalling alters the immuno-phenotype of HSC, their analysis during acute hematopoietic perturbation remains challenging and findings of stem cell activity during stress hematopoiesis vary greatly. To circumvent this problem, we employed two mouse models capable of recording HSC differentiation and proliferation in a native environment and show that the activity and output of HSCs is only marginally changed in response to physiological hematopoietic perturbations. While myeloablative chemo- or radiotherapy significantly stimulated HSC contribution by accelerating proliferation and differentiation, we did not observe alterations of HSC output into mature blood cells under conditions mimicking thrombocytopenia or microbial infections. Immunological training, in which HSCs have been proposed to store and integrate information on previous innate immune encounters, did not alter HSC proliferation and differentiation upon secondary stimulation of innate immunity. In sum, HSCs only marginally accelerated their output to immediate progeny in response to hematopoietic perturbation. Therefore, these cells do not represent a reserve population that directly participates in regeneration of mature blood cells. Instead, we postulate that the core function of adult HSCs may reside in slow, but continuous rejuvenation of extensively dividing progenitor populations and thus ensuring genomic integrity.