Prostaglandin F2 alpha concentrations in cerebrospinal fluid (CSF) from normal human subjects and patients with various pathological disorders of the central nervous system (CNS) were measured by radioimmunoassay. The mean PGF2 alpha level in 54 controls with no evidence of organic CNS disease was 67 pg/ml (range: 25-150 pg/ml). A significant increase of PGF2 alpha levels was demonstrated in most samples from patients with CNS diseases. Extremely high values were found in patients with stroke and subarachnoid hemorrhage when samples were collected shortly after the cerebral attack. With the regression of clinical symptoms and radiological findings a decrease of PGF2 alpha levels was demonstrated in this group of patients. In 32 patients with cerebral transient ischemic attacks a mean PGF2 alpha concentration of 170 pg/ml (range: 35-355 pg/ml) was found. Increased PGF2 alpha levels were found in patients with epilepsy when samples were collected within a few days after a convulsion. PGF2 alpha levels of four patients with slow progredient forms of multiple sclerosis without clinical symptoms at the time of sample collection were not different from normal controls while the mean PGF2 alpha level of all other patients with multiple sclerosis was 152 pg/ml (range: 55-325 pg/ml). Moreover, increased values could be demonstrated in patients with cerebral tumors and inflammatory processes.