: Numerous studies of melatonin, by now widely acknowledged as a circadian rhythm-affecting neurohormone, also describe its anti-oxidant, anti-cytotoxic or immune-modulating activity. While emphasizing the multifunctional aspect of melatonin action, this review presents the results of our thirty years of research, which point to the following conclusions: melatonin is capable of promoting platelet production by megakaryocytes, of acting on the latter's ion channels by way of the outward currents, and of performing a physiological anti-aggregation function thus lengthening platelet life span. Melatonin can be transported everywhere by platelets and, thanks to its lipophilicity, can cross cellular membranes easily, thus regulating blood-tissue exchanges and ensuring an improved haematic crisis. It interacts with endothelial cells by regulating their release of both relaxing-factor and contracting-factor, and with platelets by affecting their discharge of dense-body components. Finally, platelets could behave as mobile and itinerant serotonergic and/or melatonergic elements, a function comparable to the release of neurotransmitters by neurons of the central nervous system. This dynamism in melatonin physiology could prove to be a key in approaching tumour aetiopathogenesis.