Mesencephalic raphe nuclei were transplanted via the cisterna magna near the locus coeruleus. The transplants were observed at different time intervals ranging from 24 h to 4 months. Microscopic observations revealed that from 3 weeks to 4 months the transplant developed with a well-defined neural organization. Radioautographic results showed that some of the neurons in the implant at 3 weeks captured and retained [3H] serotonin. These results indicate that the intact IVth ventricle seems to be a likely culturing floor for mesencephalic raphe nuclei.