We studied N20 and N30 waves of Somatosensory Evoked Potentials from median nerve stimulation in different pharmacological conditions. N30 wave amplitude was decreased in 33 parkinsonians without therapy in comparison with a group of age-matched normal subjects. In a group of 19 parkinsonians, N30 wave amplitude was significantly augmented during apomorphine infusion and less evidently, but still significantly, during chronic 1-dopa therapy. The administration of an oral dose of haloperidol in 11 normals did not affect significantly the studied parameters. The infusion of apomorphine in 6 psychotic patients with extrapyramidal symptoms secondary to long-term treatment with neuroleptics, determined, together with a clear-cut clinical amelioration, a significant increase of N30 amplitude and N30/N20 ratio. Possible pathophysiological hypothesis of such electrophysiological modifications are discussed.