The role of GABA_A and GABA_B receptors in modulation of excitatory synaptic transmission between motoneurons and terminals from dorso-lateral funiculus (DLF) was studied in in-vitro spinal cord slices of adult turtles. Muscimol -a GABA_A receptor agonist- depressed the monosynaptic excitatory postsynaptic potential (EPSP) induced by stimulation of the DLF, and shortened its duration. The input resistance and the membrane time constant also were strongly reduced. The input membrane resistance, the amplitude and the halfwidth of the EPSP were reduced at the same rate in the presence of muscimol. Bicuculline -a GABA_A receptor antagonist- increased the EPSPs amplitude and the input membrane resistance. The EPSP amplitude ratio elicited by a paired pulse protocol did not change significantly. Our results suggest that a) muscimol acts mainly by activation of postsynaptic GABA_A receptors located on the motoneuron, and b) the synaptic strength on motoneurons may be modulated by tonic activation of postsynaptic GABA_A receptors. Baclofen -a GABA_B receptor agonist- also depressed DLF-motoneuron synaptic transmission. However, it did not affect the falling phase of the EPSPs or the motoneuron membrane time constant but induced a small decrement in input resistance. In the presence of baclofen, the amplitude ratio produced by a paired pulse protocol increased significantly. This suggests that baclofen decreased the synaptic strength by inhibition of neurotransmitter release from the DLF terminals via activation of presynaptic GABA_B receptors.