Thirty-four New Zealand white rabbits were treated with doxorubicin and imaged weekly with Tc-99m pyrophosphate to define the value of abnormal myocardial images in predicting doxorubicin-induced cardiac toxicity. Increased myocardial uptake was detected in most animals on sustained treatment with doxorubicin. A greater proportion of the heart was involved with doxorubicin-related histologic changes in animals with strongly positive myocardial images than in treated animals with moderately positive or normal scans. The myocardial images returned to normal levels 2--6 wk after doxorubicin was discontinued. Five of seven rabbits that received doxorubicin after they had three moderately positive myocardial scans, died from congestive heart failure. Three rabbits whose doxorubicin was discontinued because of scan findings, survived for 6 wk or more before dying from renal failure. The three rabbits who received the highest total dose of doxorubicin died of renal failure without developing abnormal myocardial scans.