The concentration of substance-P-like immunoreactivity (SPLI) and somatostatin-like immunoreactivity (SLI) in the lumbar spinal fluid of senile parkinsonian patients (mean age 77.6 ± 6.7 years) and senile control patients (mean age 83.5 ± 5.6 years) were determined by specific radioimmunoassays. Mean SPLI and SLI levels in the control group were 8.1 ± 2.0 (SD) and 32.5 ± 12.0 fmol/ml, respectively. The mean SPLI levels were not significantly different in the groups. The mean SLI level was significantly lower in the group of patients with Parkinson’s disease (19.8 ± 9.0 fmol/ml). A comparison with results in patients with senile dementia of Alzheimer type (SDAT) shows that, in addition to clinical and pathological correlations, Parkinson’s disease of late onset may share a deficit in somatostatinergic neuromodulation w