Ultrasound contrast agents (UCAs) microbubbles (MBs) can be preloaded with a therapeutic agents to achieve high efficiency for US-triggered drug delivery. Here we use fluorescence labeling as a substitute for theraputic drug, and use ultra high-speed fluorescence imaging for time-resolved observation of the drug release. Two configurations of drug delivery vehicles were investigated-I) lipid-shelled (unloaded) MBs and II) liposome-loaded (loaded) MBs. Different release phenomena were observed. The dynamics of release was found to be strongly dependent on ultrasonic parameters and on the MB's shell properties. MBs shrinkage following US exposure was analyzed and it indicated close correlation with the fluorescence release. This study provides valuable insights into the drug release mechanisms for ultrasound-controlled drug delivery.