The concentrations of somatostatin-like immunoreactivity (SLI) and substance-P-like immunoreactivity (SPLI) in lumbar spinal fluid of patients with senile dementia of the Alzheimer type (SDAT), multi-infarct syndrome, communicating hydrocephalus and control patients were determined by specific radio-immunoassay. Mean SLI and SPLI levels were significantly lower in an aged control patient group (mean age 83.5 +/- 5.6 years) than in an adult control patient group (mean age 30.8 +/- 10 years). In the latter group SPLI levels correlated negatively with age. Mean SLI levels decreased with deterioration in SDAT patients by up to 33% in late dementia. SPLI correlated with SLI in SDAT patients but was decreased significantly only in late dementia patients. Moderate and insignificant decreases of SLI were observed in patients with multi-infarct syndrome or communicating hydrocephalus. Analysis of SLI by gel-permeation chromatography revealed molecular heterogeneity of SLI. At least four peaks of SLI were eluted, two of which had apparent molecular weights of about 10,000 and 15,500, possibly representing somatostatin precursors. The ratio of SRIF to SLI of higher molecular weight was increased in patients with dementia compared to control patients.