The cilioexcitatory effect of 5-HT (serotonin) on lateral cilia has been observed in Mytilus. In the presence of exogenous 5-HT. K ions enhanced the cilioexcitatory effect of 5-HT after a 3 s lag period. Addition of BOL (bromolysergic acid: 5-HT inhibitor decreased the cilioexcitatory effect of 5-HT. An increase in the lag period was also observed. Addition of PCPA (p-chlorophenylalanine; tryptophan hydroxylase inhibitor) decreased endogenous stores of 5-HT and the cilioexcitatory effect of K ions. An increase in the lag period was also observed. The present investigation shows that K ions have a cilioexcitatory effect on lateral cilia after producing a momentary lag period. Furthermore, the presence of 5-HT is required to bring about the cilioexcitatory effect of K ions.