Electrophysiological investigations have been carried out in 46 patients with essential blepharospasm and 41 with hemifacial spasm. In both diseases reproducible pathologic alterations in the blink reflex were found. In essential blepharospasm a direct R1/R2 transition and a contralateral early component with and without direct transition into the R2C component were recorded. In patients with hemifacial spasm there was a loss of the R1 component in addition. In repeated treatments with botulinum toxin the amplitude (R1,R2) of the blink reflex was reduced in some cases, although in some cases the intensity of spasm was as high as before treatment once the effect had faded.