The pineal gland plays a role in the seasonal re-organization of the circadian system that occurs in Ruin lizard Podarcis sicula. In fact, either pinealectomy or melatonin implants induce an immediate transition from the circadian locomotor pattern typical of summer to the circadian locomotor pattern typical of spring or autumn. Furthermore, the degree of involvement of melatonin and the pineal in circadian organization changes dramatically with season: 1. The effects of pinealectomy, as well as those of melatonin implants on circadian locomotor rhythms are strong in summer and weak in the other seasons; 2. Daily melatonin injections entrain locomotor rhythms in summer, but not in other seasons; 3. A phase response curve for melatonin was generated in summer, but not in winter; 4. Daily injections of luzindole, an antagonist for melatonin receptors alter locomotor rhythms in summer, but not in other seasons; 5. A robust pineal-dependent circadian rhythm of circulating melatonin exists in summer, but not in spring or in autumn. This confirms that the central role of the pineal in determining circadian organization in summer and the marginal role of the pineal in either spring or autumn are respectively due to appearance (summer) or disappearance (spring, autumn) of a circadian melatonin signal in the circulation.