Several neuroactive compounds have been implicated as playing roles in the circuitry that controls larval metamorphosis in marine molluscs. For the caenogastropod Ilyanassa obsoleta, results of neuroanatomical studies suggest that the production of nitric oxide (NO) increases throughout the planktonic stage and that NO production is necessary for the maintenance of the larval state, especially as it becomes metamorphically competent. Bath application or injection of exogenous serotonin (5HT) can initiate metamorphosis in competent larvae, and exogenous NO can inhibit such serotonergically-induced metamorphosis. Inhibition of endogenous nitric oxide synthase (NOS) can also trigger larval metamorphosis. The production of endogenous NO appears to decrease concurrently with the initiation of metamorphosis, but the specific interactions between serotonergic and nitrergic neurons are unknown. Evidence in support of NO acting to up-regulate the enzyme guanylyl cyclase (GC) is still equivocal. Thus, we do not yet know if NO exerts its effects through the actions of cyclic 3′,5′-guanosine monophosphate (cGMP) or by a cGMP-independent mechanism. The ubiquity of nitrergic signalling and its significance for developing molluscan embryos and larvae are still the subject of speculation and require further investigation.