The work was carried out to investigate the effects of clonidine on Ach quantal release of presynaptic nerve terminals in guinea pig celiac ganglia in vitro by means of intracellular recording technique. After perfusion of celiac ganglia with clonidine ( 10?mol/L ) for 7 ~10 min, the frequency of spontaneous miniature excitatory postsynaptic potentials ( mEPSP ) which was increased by high K+ (15 mmol/L ) , diminished about 35% with little change in amplitude. Under condition of low Ca2+( 0.5mmol/L )/high Mg2+( 5.5mmol/L ) presynaptic nerve was repetitively stimulated by 200 pulses ( 1 Hz), the failure number of EPSP increased from 82 to 145 in 200 stimuli and the quantal content ( m ) decreased about 62% with little change in quantal size ( q ) under clonidine ( 10?mol/L ) action. It is indicated that clonidine can decrease Ach auantal release from presynaptic nerve terminals without effecting Ach content in vesicles, and is one of the mechanisms for depressant effect of clonidine on synaptic transmission of the sympathetic ganglia.