1. The acute effects of neurotrophic factors on the frequency of spontaneous transmitter release (miniature endplate potentials (MEPPs)) from motor nerve terminals has been examined in skeletal muscles of neonatal mice aged between 9 and 20 days. The following factors were tested at a concentration of 50 ng ml-1: brain-derived neurotrophic factor (BDNF), neurotrophin-3 (NT-3), neurotrophin-4 (NT-4), ciliary neuronotrophic factor (CNTF), leukaemia inhibitory factor (LIF), insulin-like growth factors 1 and 2 (IGF-1 and IGF-2), and glial cell line-derived neurotrophic factor (GDNF). In some experiments, the responses to 2 microM LaCl3 and 10 mM K+, or to 2-5 nM purified alpha-latrotoxin (alpha-LTX) were also measured. 2. Neither BDNF, NT-3, NT-4, LIF, IGF-1 or IGF-2 - singly or in combination - caused any significant change in MEPP frequency. GDNF, however, produced a highly significant, 2-fold increase in neurotransmitter release that was reproduced in fourteen muscles. 3. Potentiation of MEPP frequency in GDNF was of the same order as that induced by tetanic stimulation or substitution of the bathing medium with hypertonic saline; but substantially less than that induced either by lanthanum ions or alpha-latrotoxin. 4. The data suggest that concentrations of GDNF that produce maximal enhancement of motoneurone survival in vitro and in vivo also produce acute, non-saturating enhancement in transmitter release at immature mammalian neuromuscular synapses. Taken together with other reports, these findings suggest that GDNF may mediate both functional and structural plasticity of neonatal neuromuscular junctions.