In six dogs with botulism type C electrophysiological examinations showed: fibrillation potentials and prolonged insertional activity; low amplitude of the evoked muscle action potential; decrease in amplitude of the compound muscle action potential with slow repetitive stimulation; slowing of motor and sensory velocities in the peripheral nerve; and restoration of velocity and amplitude corresponding to clinical improvement. These findings indicate peripheral nerve dysfunction which cannot be explained adequately by current knowledge of the action of botulinum toxin on cholinergic nerve endings. It is therefore suggested that botulinum toxin also interferes with peripheral nerve conduction.