The oxygen consumption of a single ascidian larva was measured. After hatching the consumption increases gradually. During the period of tail resorption it also increases gradually, but after the completion of tail resorption the consumption decreases conspicuously. With the development of the larva after hatching, the activities of cytochrome oxidase and succinic dehydrogenase and of Janus green-reduction become detectable in the adhesive papillae, the proximal region of the tail, and the tail muscle. After the completion of tail resorption, these activities become indistinct. These tissues underwent most profound morphological changes at the onset of metamorphosis. Soon after hatching, Janus green has no effect to induce metamorphosis. In larvae 4 hr after hatching, the shrinkage of adhesive papillae can be induced by Janus green-treatment. In 12 hr larvae, both the shrinkage of adhesive papillae and the tail resorption can be induced by Janus green. The enhancement of respiratory activities in the larvae after hatching may be related to the changes in the adhesive papillae and later to changes in the proximal region of the tail. Only when both of these changes occur can metamorphosis be induced.