1. The ability of different neurotrophic factors to maintain and regulate synaptic function at the developing motoneuron was studied in Xenopus nerve-muscle co-cultures. Spontaneous synaptic currents (SSCs) were measured by using whole-cell voltage-clamped myocytes. 2. Compared with natural synapses, motoneurons without contact on a myocyte (naive neurons) released ACh in smaller quantal packets, the amplitude being inversely proportional to the days in culture. The mean SSC amplitudes of naive neurons, which were measured by manipulating a myoball into contact with the myocyte-free nerve terminals to form a manipulated synapse, were 99.5 +/- 6.7 and 48.2 +/- 1.9 pA for day-1 and day-3 cultures, respectively. 3. Chronic treatment of day-1 cultures with brain-derived neurotrophic factor (BDNF), neurotrophin-3 (NT-3), neurotrophin-4 (NT-4), ciliary neurotrophic factor (CNTF) or glial cell line-derived neurotrophic factor (GDNF) for 2 days, increased the ACh quantal size of naive motoneurons in a concentration-dependent manner, whereas insulin-like growth factor-1 (IGF-1) and basic fibroblast growth factor (bFGF) had no effect, even at high concentrations. 4. The interaction of various neurotrophic factors was examined, using concentrations that gave maximal effects. Combination of CNTF plus BDNF or CNTF plus NT-3 had synergistic effects in potentiating SSC amplitude of the manipulated synapse of naive neurons, whereas NT-3 plus BDNF, NT-3 plus GDNF, BDNF plus GDNF or CNTF plus GDNF had no synergistic action. 5. Chronic treatment with d-tubocurarine for 2 days resulted in a reduction of the quantal size of natural synapses. Concomitant treatment with BDNF, NT-3, GDNF, CNTF but not bFGF or IGF-1, reconstituted the SSC amplitude. 6. Taken together, these findings suggest that BDNF, NT-3, NT-4, CNTF and GDNF may regulate and maintain the synaptic function of developing motoneurons, and different neurotrophic factors utilizing distinct signalling mechanisms may have synergistic actions.