The shortacting bar-biturate, thiamylal sodium, was injected intravenously in doses of 10 mg/kg to cats lightly anesthetized with pentobarbital sodium, immobilized with flaxedil and kept on artificial respiration, and the effects on the monosynaptic spinal reflex studied. Thiamylal decreased the focal potential recorded extracellularly from the motor nucleus, including both the presynaptic and postsynaptic component, the excitatory postsynaptic potential recorded intracellularly from the motoneurons, and the potential recorded from the ventral root. No changes were observed in the potential recorded at the dorsal root entry or in the intracellularly recorded resting potential, spike potential, firing threshold, electrical properties and accomodation of the motoneurons, and the excitabilities of the afferent terminals and the motoneuron somas were unaltered. Pentobarbital had the same effects. It is postulated that barbiturates in these doses act mainly on the afferent nerve terminals resulting in less transmitter release and reduced synaptic potential.