It was shown that local application of agonists of the 3rd type receptors 5-HTQ and quipazine into the interblastomere cleft of Paracentrotus lividus embryos evoked specific membrane currents. At the same time, ligands of 5-HT₃-receptors specifically affected the cleavage patterns of half-embryos, i.e., imitated or avoided the interblastomere signal. In the view of the data obtained, we discuss a more precise concept of protosynapse, where the distribution of membrane serotonin receptors is restricted to the period of blastomere formation during cleavage and localized in the area of interblastomere contact.