Metamorphosis is the phenomenon of irreversible transformation from one life history stage to another. A number of fundamental changes occur during this process, which are mediated by complex signal transduction mechanisms. Existing findings have established that several signaling molecules could influence the metamorphosis in marine animals. For example, activating protein kinase C induces metamorphosis, while increasing nitric oxide level is generally believed to have an opposing effect. However, the order of these biological signals was seldom explored. In the current research, pharmacological and imaging experiments were conducted to demonstrate the relationship between nitric oxide and protein kinase C during metamorphosis of marine gastropod larvae, Crepidula fornicata. PMA treatment established that elevating protein kinase C activity induced metamorphosis in C. fornicata larvae. A double treatment experiment with protein kinase C inhibitor, H-7 and nitric oxide synthase inhibitor, SMIS, showed that PKC inhibition delayed metamorphosis induced by nitric oxide depletion. On the other hand, exogenous nitrci oxide generated by sodium nitroprusside did not maintain larval state, implying that exogenous nitric oxide functioned differently from endogenous nitric oxide. Diaminofluorescein staining labeled a specific structure in all competent larvae and some juveniles, which might be associated with metamorphosis.