Using a radioimmunoassay procedure substance P-like activity was measured in samples of human CSF obtained from 12 patients with major depressive disorder, 12 with schizophrenia, and 15 control cases diagnosed as psychiatrically normal. Levels were significantly higher in CSF of patients with depressive disorder as compared with schizophrenic patients or controls. Chemical characterization revealed that the measured activity was less basic than substance P itself and might be due to C-terminal fragments. A component reacting with an antiserum highly specific for the substance P[1-7]fragment was found in CSF of patients with depressive disorder. The results indicate that substance P-related peptides may be biological markers in psychoses, particularly in major depressive disorder.