THE effects of BDNF, and NT-4/5 on AMPA receptor-mediated synaptic transmission were investigated with the patch clamp technique applied to embryonic, and postnatal rat hippocampal neurones, cultured in serum-free medium. Spatially restricted application of neurotrophin-containing solution on to the recorded cells was performed, and evoked as well as miniature excitatory postsynaptic currents (mepscs) were monitored. In approximately 25% of neurones tested a transient augmentation of evoked synaptic currents, and a transient increase in the frequency of mepscs occurred with a delay of 0.5–5 min after the onset of BDNF or NT-4/5 application. The amplitudes of the AMPA receptor mediated mepscs were unaffected, suggesting a presynaptic action of BDNF, and NT-4/5.