Valproic acid (VPA), an antiepileptic drug and HDAC inhibitor, has been identified as a drug candidate for spinal muscular atrophy (SMA), a motoneuron disorder for which currently no effective therapy is available. Based on its potential to up-regulate SMN expression from the SMN2 gene in fibroblasts and lymphoblastoid cell lines from SMA patients, we analysed the effects of VPA in isolated motoneurons from Smn(-/-);SMN2 mice, a model for SMA type I. Treatment with VPA increased Smn expression but unexpectedly also led to reduced growth cone size and reduced excitability in axon terminals of mutant motoneurons. Analysis of Ca2+ currents and distribution of voltage-gated Ca2+ channels revealed an inhibitory function of VPA on voltage-gated Ca2+ channels and possibly also other ion channels that contribute to presynaptic excitability of motoneurons. Our data indicate effects of VPA which might aggravate disease-specific symptoms in SMA patients.