The threshold for corticomotor excitation by magnetic stimuli is a clinically important parameter in the assessment of central motor conduction. Hitherto, arbitrary criteria have been adopted to define threshold and there are few published data using a figure of eight coil. A new method is described in which the highest stimulus intensity evoking responses in the relaxed first dorsal interosseous muscle with a probability of zero [lower threshold (LT)] and the lowest intensity evoking responses with a probability of one [upper threshold (UT)] have been determined. The method, which utilizes a coil located and optimally oriented over hand motor cortex, minimizes the number of stimuli needed. Both LT and UT are normally distributed and are independent of age, gender, and hemisphere. In 102 healthy hands of 55 control subjects 23-84 years of age, mean +/- SD UT was 46.6 +/- 9.4% and LT was 38.0 +/- 8.6% maximum stimulator output. Repeatability estimates indicated an absolute change of more than 13% or 11% in UT or LT respectively in the same individual over 1-3 months was significant at the 5% level.