The acute effects of intravenous melatonin on spontaneously active striatal neurons in sham-operated and pinealectomized rats were studied. Extracellular recordings in a total of 76 neurons showed that only 19 did not modify their spontaneous activity after melatonin injection. In sham-pinealectomized rats, neural firing decreased in most cells (80% of neurons), and increased in only 5.7% of the neurons after indole administration (100 ng/kg body weight). However, in the group of rats pinealectomized 7 days earlier, the injection of melatonin (at the same dose as above) significantly increased the excitatory response (44%), while the number of cells showing inhibitory response decreased (17%). Moreover, a small percentage (4.9%) of neurons in pinealectomized rats displayed a biphasic response (initial decrease followed by an increased firing). These results demonstrate that aMT can modulate the activity of striatal neurons, and suggest that other compounds of pineal origin (e.g., vasotocin) may change effects of aMT on basal ganglia neurons.