Riluzole, the only drug available againsta myotrophic lateral sclerosis (ALS), has recently been shown to block muscle ACh receptors (AChRs), raising concerns about possible negative side-effects on neuromuscular transmission in treated patients. In this work we studied riluzole's impact on the function of muscle AChRs in vitro and on neuromuscular transmission in ALS patients, using electrophysiological techniques. Human recombinant AChRs composed of alpha(1)beta(1)delta subunits plus the gamma or epsilon subunit (gamma- or epsilon-AChR) were expressed in HEK cells or Xenopus oocytes. In both preparations, riluzole at 0.5 mu M, a clinically relevant concentration, reversibly reduced the amplitude and accelerated the decay of ACh-evoked current if applied before coapplication with ACh. The action on gamma-AChRs was more potent and faster than on epsilon-AChRs. In HEK outside-out patches, riluzole-induced block of macroscopic ACh-evoked current gradually developed during the initial milliseconds of ACh presence. Single channel recordings in HEK cells and in human myotubes from ALS patients showed that riluzole prolongs channel closed time, but has no effect on channel conductance and open duration. Finally, compound muscle action potentials (CMAPs) evoked by nerve stimulation in ALS patients remained unaltered after a 1 week suspension of riluzole treatment. These data indicate that riluzole, while apparently safe with regard to synaptic transmission, may affect the function of AChRs expressed in denervated muscle fibres of ALS patients, with biological consequences that remain to be investigated.