Although during development the circulatory system typically starts operating earlier than any other organ, blood flow apparently is not linked to metabolic requirements of tissues in early embryonic and larval stages of fish and amphibians. Nevertheless, even at this stage, environmental factors like hypoxia can modify cardiac activity, induce a redistribution of blood flow and stimulate erythropoiesis in zebrafish larvae. Similarly, an increase in local oxygen demand induced by prolonged swimming activity induces physiological adaptations resulting in an enhanced swimming efficiency, and in an increased tolerance towards hypoxia. In consequence, even during early development of fish larvae the performance of cardiac muscle and of skeletal muscle can be modified by environmental influences, and peripheral resistance can be adjusted. At this stage the control appears to be possible by local and hormonal components, but not by the autonomous nervous system. The influence of environmental factors, however, is not restricted to the physiological performance of the cardio-vascular system, it also extends to the structural development. At the tissue level prolonged exercise results in an increase in mitochondrial density in red muscle fibers, and an increase in capillary density. Supported by the Austrian Science Foundation (FWF, P12571BIO, P14976-BIO)