In parallel with improvements in understanding pain neurophysiology, many chemicals have recently been investigated for spinal anaesthesia and analgesia. However, studies discussing the effects of these drugs on neural tissue indicate that knowledge about some aspects of neurotoxicity is limited. Forty-nine New Zealand albino rabbits, weighing 2.2+/-0.2 kg, were randomly assigned to seven groups of seven animals each. Single dose groups received intrathecally through the atlantooccipital membrane 0.9% saline 1.5 ml; midazolam 100 mu g/kg (low dose midazolam group) or 500 mu g/kg (high dose midazolam group); neostigmine 10 mu g/kg (low dose neostigmine group) or 50 mu g/kg (high dose neostigmine group). Two groups had seven days of repeated dosing with either midazolam 100 mu g/kg/day (repeat midazolam group) or 10 mu g/kg/day neostigmine (repeat neostigmine group). The animals were sacrificed on day 8, and two spinal cord sections from the fourth cervical level and fourth lumbar level were removed and prepared for histopathological study. Transmission electron microscopic evaluations were performed on transverse spinal cord sections by a neuropathologist blinded to the group allocation. Twenty myelinated axons and neurones in the cervical and lumbar sections were investigated for the histopathological study. This study indicates that midazolam and neostigmine have different neurotoxic effects that depend on the dose and the repetition of dosing when these drugs arc, administered intrathecally.