Neurosurgical apparatus is described that comprises a guide device (60) and a neurosurgical instrument (40). The guide device includes a tube (66) for insertion into the brain of a subject and a head (64) attached to the proximal end of the tube (66) for affixing the guide device (60) to a hole formed in the skull. The head has a passageway (70) therethrough in communication with the bore of the tube (66) such that the bore of the tube and the passageway (70) through the head (64) define an internal channel through which a neurosurgical instrument (40) can be passed into the brain (10) of the subject. The neurosurgical instrument (40) such as a catheter is for insertion to a desired brain target through the internal channel of the guide device (60). The apparatus comprises one or more sealing elements (50 72) for providing a substantially fluid tight seal between the internal channel of the guide device (60) and the exterior of the neurosurgical instrument (40) when inserted therein. This prevents fluid leakage from guide tube.
