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Pinealectomized lizards (Podarcis sicula) whose locomotor rhythms were recorded in constant temperature (29 [degrees] C) and darkness were subdivided into 2 groups of hosts: Each belonging to the 1st group (experimentals) received from a donor a pineal gland, and each belonging to the 2nd one (controls) received a piece of cerebellum. Pineal transplantation induced drastic changes in the free-running period ([Tau]) of locomotor rhythms, which were significantly greater than the [Tau] changes induced by cerebellum transplantation. Either application or removal of melatonin implants left the locomotor rhythms of the controls completely undisturbed, showing that in absence of melatonin rhythms (pinealectomy alone abolishes blood-borne melatonin rhythms) melatonin implants are ineffective. Melatonin rhythms, however, had to be present in the experimentals, because the circadian system reacts to melatonin implants by changing [Tau] (as if melatonin rhythms had been suppressed) and to removal of the implants by again changing [Tau] (as if melatonin rhythms had been restored).