Measurements of human chorionic gonadotrophin (HCG) concentration in plasma and cerebrospinal fluid (CSF) have been made on patients with gonadotrophin-producing tumors. In the absence of brain metastases the spinal fluid concentration is, within wide limits, proportional to that in the plasma. In 73 patients with gestational choriocarcinoma the mean plasma/spinal fluid ratio was 286 with a lowest value of 64. In five patients with gonadotrophin-producing teratomas the mean ratio was 208 with a lowest value of 104. In 29/33 patients with brain metastases the plasma:CSF ratio was less than 60 at the time confirmatory evidence of brain metastases was obtained. One patient with a gonadotrophin-producing teratoma had a brain metastasis that apparently failed to produce HCG and this metastasis failed to show the histologic features of choriocarcinoma. Monitoring the plasma:CSF ratio provided evidence of brain metastases in 13/18 patients who were undergoing chemotherapy for extensive metastatic disease before confirmatory evidence was obtained by other methods. The lead-in time between assay diagnosis and diagnosis by other methods ranged from 1-20 weeks. Monitoring the plasma:CSF ratio provided a means of observing the response of cerebral metastases to therapy. In some patients the CSF HCG concentration exceeded the plasma concentration indicating that the higher CSF values in patients with CNS metastases cannot be attributed to impairment of the blood-brain barrier. Direct secretion of tumor products into CSF or indirect secretion into CSF via cerebral extracellular fluid evidently occurs. In contrast with radiographic and radionucleide detection methods, a chemical marker indicates the metabolic activity of tumor cells within the central nervous system and provides a basis for monitoring that activity.