Previous work has shown that niacin deficiency in rats increases the severity of ethylnitrosourea (ENU)-induced anemia and leukopenia and the long-term development of cancer. The current study was initially designed to characterize changes in bone marrow cell populations during ENU treatment in this model. Weanling Long-Evans rats were fed diets containing 0 or 30 mg/kg of added niacin for a period of 2-3 wk. ENU treatment started after 1 wk of feeding and consisted of either 4 or 8 doses of ENU delivered by gavage, every other day. Niacin deficiency (ND) alone caused a significant depression in nucleated red blood cells (30%), and a sporadic effect on granulocytes (+23% after 4 doses of vehicle, -29% after 8 doses of vehicle). ENU treatment, after only 4 doses, caused a large decline in the numbers of bone marrow cells, and this effect was enhanced by ND (ENU decreased lymphocytes by 66% in pair-fed (PF) and 86% in ND, granulocytes by 41% in PF and 64% in ND, and nucleated red blood cells by 63% in PF and 71% in ND). Cell cycle distribution suggested that bone marrow cells in niacin-adequate rats, but not ND rats, mounted a compensatory proliferative response during chronic ENU exposure. ND alone caused an 80% decrease in bone marrow NAD+ levels at all time points. Surprisingly, chronic exposure to ENU (which should cause DNA damage and NAD+ utilization) led to a 2.8-fold increase in NAD+ content in ND marrow cells. This finding led to a second study in which ND and niacin-adequate PF control rats received 7 doses of ENU or vehicle (CON), after which all rats received 1 dose of ENU. In this study, modestly enhanced bone marrow NAD+ in chronically treated PF rats was used to synthesize 2-fold greater amounts of poly(ADP-ribose) than seen after one acute dose of ENU, while this did not occur in chronically treated ND rats, in spite of a 2.8-fold increase in bone marrow NAD+. This study has shown that bone marrow cell populations are sensitized to ENU treatment by ND, that NAD+ pools are regulated in response to DNA damage, and that NAD+ localization and/or utilization in the nucleus is altered during ND and chronic DNA damage.