S U M M A R Y Using a collision technique, the ulnar nerve was made refractory for a shorter distance normally covered in 0.5 ms or a longer distance covered in 1.5 ms. Studying the shorter refractory segment with paired shocks of maximal intensity, the test response first appeared (more than 5% of unconditioned response) at an interstimulus interval of 1 After passage of an impulse, an axon becomes totally inexcitable for a fraction of a millisecond, followed by progressive recovery to prestimulus level within the ensuing few milliseconds. These two phases of decreased excitability, the absolute and relative refractory periods, have been studied in experimental animals by measuring the nerve action potentials elicited by paired stimuli (Bishop and Heinbecker, 1930;Graham, 1935;Tasaki, 1953;Hodgkin, 1967;Bergmans, 1973). In 1963, Gilliatt and Willison reported the first comprehensive study on the refractory periods of human peripheral nerves. This and subsequent human studies, however, have been limited to sensory and mixed fibres (Buchthal and Rosenfalck, 1966;Lowitzsch et al., 1973;Hopf et al., 1974;Tackmann and Lehmann, 1974;Betts et al., 1976;Hopf et al., 1976