Immunoreactive neurokinin A was measured in the cerebrospinal fluid of twelve patients with Parkinson's disease and eleven normal subjects, using a sensitive and precise extraction/concentration radioimmunoassay method. The mean value obtained in Parkinson's disease patients (13.2 +/- 4.6 pmol/l) was lower than that of the controls (17.4 +/- 5.9). The tendency toward a significant decrease (p = 0.085) found in this preliminary study could indicate that neurokinin A containing neurons are involved in the pathophysiology of Parkinson's disease. In addition, the establishment of reference values for neurokinin A in cerebrospinal fluid may provide a basis for further studies of this neuropeptide in neurological disorders.