Diazepam seems to have no direct effect on the excitability of spinal motor neurons in the rat. Depression of reflexes in animals with intact mesencephalic-spinal connections is consistent with depression of mid-brain excitatory motor pathways. Very high doses of diazepam (10–12 mg/kg) reverse these effects in intact animals, possibly by depression of inhibitory neurons. The solvent, propylene glycol, is not responsible for any of these effects.