[The characteristics of the synchronizing action of melatonin on the dynamics of circadian mobility in rats].

Morning (9.00-10.00 h), but not noon, repeated injections of the pineal hormone melatonin (1 or 10 mg/kg daily, during 2 weeks) synchronize the circadian locomotor rhythm of rats. As shown with inverted photoperiod, melatonin facilitates off and on light reactions. Some animals have low and some high hormonal susceptibility. This indicates the role of injections regime and rat's individual reactivity for expressiveness of melatonin synchronizing effect.