Isolated spinal cord preparations from Rana ridibunda frogs were used for studies of the effects of the GABA(B) receptor agonists (-)-baclofen (50 and 100 microM) and GABA (4-8 mM) and the specific GABA(B) receptor antagonist 2-hydroxysaclofen (100 microM) on the transmission of signals from fibers of the ventral columns monosynaptically connected with motoneurons in segments 9 and 10. These experiments showed that (-)-baclofen (50 and 100 microM) produced significant and dose-dependent suppression of excitatory postsynaptic potentials (EPSP) in motoneurons and ventral root potentials evoked by stimulation of fibers of the ipsi- and contralateral ventral columns. The inhibitory effect of (-)-baclofen (100 microM) on descending EPSP was 35-50% blocked by the GABA(B) receptor antagonist 2-hydroxysaclofen (0.2 mM). The inhibitory effect of GABA (4-8 mM) on descending EPSP was 60% blocked by the GABA(A) receptor antagonist picrotoxin (0.05 mM). (-)-Baclofen (50 microM) and GABA (4 and 6 mM) were found to have inhibitory effects on ventral root potentials evoked by stimulation of the ipsi- and contralateral ventral columns. The data obtained here, as well as data obtained by pharmacological analysis and conditioning by stimulation of the ipsi- and contralateral ventral columns, are regarded as a significant argument supporting the existence of GABA(B) receptor-mediated presynaptic inhibition of descending fibers connected monosynaptically to spinal cord motoneurons in the frog Rana ridibunda.