Muscimol, ibotenic acid, isoguvacine, 4,5,6,7-tetrahydroisoxazolo [5,4-c]pyridin-3-ol (THIP) and gamma-aminobutyric acid (GABA) injected intravenously exerted the following dose-dependent effects on the lumbosacral spinal cord of spinal cats: 1. The excitability of primary afferents and the amplitude of dorsal root reflexes were enhanced; segmental monosynaptic ventral root reflexes and dorsal root potentials as well as the spontaneous gamma-fibre activity and, to a smaller extent, the excitability of motoneurons and polysynaptic reflexes, were depressed. 2. Muscimol was the most potent compound (0.3-1 mg/kg produced significant effects), followed by ibotenic acid (3-10 mg/kg), isoguvacine and THIP (10-30 mg/kg) and GABA (100 mg/kg). 3. Most effects were reversibly antagonized by bicuculline, but not by strychnine, indicating that they were due to a specific interaction with GABA-receptors. 4. These results suggest that the systemic injection of GABA-mimetic agents affects spinal cord activities by actions related to GABA-receptors located on primary afferent endings, intrinsic spinal neurons, and motoneurons.