Reverse-transcription polymerase chain reaction (RT-PCR) investigation of the expression of the components supposedly taking part in serotonin regulation of the early development of Paracentrotus lividus has shown the presence of transcripts of five receptors, one of which has conservative amino acid residues characteristic of monoaminergic receptors. At the early stages of embryogenesis the expressions of serotonin transporter (SERT) and noradrenaline transporter (NET) were also recognized. The activities of the enzymes of serotonin synthesis and serotonin transporter were shown using immunohistochemistry and incubation with para-chlorophenylalanine (PCPA) and 5-hydroxytryptophan (HTP). Pharmacological experiments have shown a preferential cytostatic activity of ligands characterized as mammalian 5-hydroxytryptamine (5-HT)1-antagonists. On the basis of the sum of the data from molecular biology and embryo physiological experiments, it is suggested that metabotropic serotonin receptors and membrane transporters take part in the regulatory processes of early sea urchin embryogenesis.