This study was undertaken in order to determine whether early administration of mannitol is different from late administration in its effect on brain oedema. Cold-induced brain oedema, which was confirmed by high resolution CT scan, was produced in 2 groups of cats. In group one mannitol was given early (90 minutes after injury); in group two 3-4 hours after the injury (late). Repeated CT scans following mannitol administration showed that the early group exhibited significantly greater dehydration (p less than 0.0001) while the late group showed significant hydration, in the lesioned hemisphere. The contralateral control hemisphere responded to mannitol with similar dehydration effect in both groups.