Chemotherapy is one of the most successful cancer treatments used today. Unfortunately, the amount of chemotherapy a patient can receive is limited by the associated negative side effects, such as cardiotoxicity, immune system suppression, and nephrotoxicity. Encapsulation of these drugs, Doxorubicin (DOX) in particular, in stabilized Pluronic micelles (Plurogel TM) shows success in limiting these harmful side effects. In previous studies, low-frequency ultrasound (US) has been shown, in vitro, to locally release DOX from these micelles. In this study, a novel drug delivery system involving the encapsulation of DOX in Plurogel and the release of the drug at the tumor site using ultrasound was studied in vivo using rats. These studies determined the effect of ultrasonically released drugs on tumor growth rate and drug delivery to the tumor tissue. Concurrently, different frequencies (20 kHz, 500 kHz) were tested for the same effects. Treatments consisted of micelle-encapsulated doxorubicin injected intravenously followed by ultrasound application to one of the two bilateral tumors. Also, in different experiments, pharmacokinetic studies of the drug in the heart, liver, leg muscle, and tumors were performed up to a period of one week after treatment. Results showed that tumors treated with ultrasound displayed, on average, slower growth rates than non-insonated tumors (P = 0.0047). Also, insonated tumors displayed a weak increased concentration of DOX than non-insonated tumors within the first eight hours after treatment (P = 0.064). However, comparison between tumors which received 20 kHz and 500 kHz ultrasound treatment showed no statistical difference (P = 0.9275) in tumor growth rate or DOX concentration. It is noteworthy that the insonated tumor has slower growth even though the amount of DOX was not that much greater in the non-insonated tumor. This suggests that US also affects the uptake and/or processing of the DOX by the tumor cells, and that the therapeutic effect may not be attributed solely to a higher concentration of drug released by insonation. Pharmacokinetic studies showed significant drug accumulation in the heart but no accumulation in the liver, skeletal leg muscle, or tumors over the course of four weeks of consecutive weekly injections of DOX-encapsulated Plurogel. After 24 hours, DOX concentration remains the greatest in the tumors, regardless of whether they received ultrasound or not.