The neurophysiological findings in eight patients with the stiff man syndrome (SMS), including four of six tested with autoantibodies against glutamic acid decarboxylase, are presented. Neurophysiological findings did not make it possible to discriminate between patients with and those without autoimmunity against GABAergic neurons. Investigation of mono- and polysynaptic reflexes revealed abnormal results in a variable number of SMS patients, the abnormalities largely corresponding to those seen in spastic paresis. A stereotyped motor response to electrical stimulation of peripheral nerves was recorded from the trunk muscles of all patients investigated. This response was termed spasmodic reflex myoclonus and consisted of a sequence of 1-3 synchronous myoclonic bursts, 60-70 ms after median nerve stimulation, followed by a tonic decrescendo activity over a number of seconds. The recruitment order of muscles along the neuraxis in spasmodic reflex myoclonus suggested that the latter was generated in the spinal cord and conveyed via propriospinal tracts. It is thought that spasmodic reflex myoclonus may serve not only as a diagnostic tool, but also as a key to understanding some aspects of the pathophysiology of both spasms and stiffness in SMS. It is speculated that stiffness is a fragment of spasms, both being generated by common neuronal mechanisms tentatively ascribed to interneurons in the spinal grey matter.