As well as acting as a neurotransmitter, serotonin is also involved in morphogenetic signalling during crucial phases of many developmental and regenerative processes such as cleavage, migration and differentiation. Echinoderms display nerve-dependent regenerative phenomena and serotonin is one of the main neural regulatory factors that have been identified in them. The present work was designed to investigate the broad-spectrum involvement of this molecule in echinoderm regeneration, focusing on arm regeneration in the crinoid Antedon mediterranea. We carried out specific in vivo exposure experiments to selected anti-serotonergic drugs (a synthesis inhibitor: parachlorophenylalanine and a receptor antagonist: methiothepin) and explored their possible effects on arm regeneration. Drug exposure appeared to affect regeneration, causing a general delay of regenerative growth and several histological anomalies mainly in the muscles of the stump. In addition, exposure to the antagonist produced a marked reduction of cell proliferation in both the regenerate and the stump. Our results provide further evidence that serotonin has wider functions than those related to interneuronal communication and that it may be a critical signalling molecule in the main processes of regeneration such as proliferation, morphogenesis and differentiation.