Apparatus for delivering therapeutic agents to the central nervous system of a subject is described. The apparatus comprises a percutaneous access device (50；140；150；160) having a body (10；52；152) with at least one extracorporeal surface (22；68) and at least one subcutaneous surface. The body (10；52) defines at least one port (12；116) for connection to an implanted intracranial catheter. The port (12；116) is accessible from the extracorporeal surface (22；68) of the device, but is provided with a seal (14；112；142；146) such as a rubber bung between the lumen of the port and the extracorporeal surface (22；68).A guide member (42) is provided that projects from the body (10；52) of the device, the guide member (42) ensuring that the percutaneous access device (50；140；150；160) and an associated delivery unit (34) can only be fitted together one way. The percutaneous access device may have more than two ports (12；116).
