The effects of lithium on the tail nerve conduction velocity of the rat were studied. No effect of lithium on the conduction velocity was found, even at toxic serum lithium levels. No effect of lithium on repetitive stimulation of the distal forelimb flexor muscles was seen. The results suggest that electrophysiological measurements do not provide an effective means of titrating serum lithium levels in the rat. The results may have clinical significance since they provide evidence against neuropathic or neuromuscular transmission defects as the cause of the transient weakness of some patients taking lithium.