This study was designed to investigate whether either of 2 dosage schedules of N-acetylcysteine (NAC) was effective in preventing chronic doxorubicin-induced heart failure in dogs. Thirty-eight dogs were randomly assigned to 1 of 4 groups: controls, 10 dogs; doxorubicin only, 12 dogs; doxorubicin + low dose NAC, 8 dogs; and doxorubicin + high dose NAC, 8 dogs. All dogs except the controls received 1 mg/kg of doxorubicin weekly for 8 weeks and then every other week for 8 weeks. The doxorubicin + low-dose NAC group received 140 mg/kg of NAC 30 minutes before each dose of doxorubicin. The doxorubicin + high-dose NAC group received NAC before and then twice a day for 5 days. Systolic time intervals and echocardiograms were obtained weekly; cardiac catheterization was performed at the conclusion of the study. Of the 38 dogs in the study, 9 died; all deaths were in the doxorubicin treatment groups. The incidence of death was not different between the doxorubicin-only, the doxorubicin + low-dose and the doxorubicin + high-dose NAC groups. The noninvasive and the invasive and the catheterization data generally revealed poorer cardiac function of the doxorubicin treatment groups than in controls. However, no significant differences existed between the doxorubicin-only and doxorubicin + low-dose and doxorubicin + high-dose NAC groups. In conclusion, NAC in a low- or high-dose regimen did not significantly ameliorate doxorubicin cardiac toxicity. Because NAC is a free radical scavenger, perhaps doxorubicin cardiac toxicity is not a result of free radical generation.