Non-invasive measurements of the 50 kHz impedance of the anterior forearm show that the resistance and reactance increase under voluntary isometric contraction of the finger flexor muscles. The relationship between impedance and force is nonlinear, dependent on the type of test, the history of prior exercise, and the health status of the subject. Nevertheless, useful dynamic response parameters betaR = deltaR/R0deltaF and betax = deltaX/X0deltaF can be defined, typically a few hundredths of a per cent per newton. Evidence is presented for the view that these effects reflect dominantly physiological as opposed to morphological changes in the muscle. In particular, (a) the impedance changes many milliseconds before the force is generated, (b) betaR and betaX change substantially during a series of repetitions of the same exercise, and (c) the impedance does not return to its original value following relaxation of the muscle. Supporting data are presented for six healthy men and women, with ages ranging from 19 to 70 years. A preliminary study of patients with various neuromuscular diseases was also performed, amongst whom marked quantitative and qualitative contrasts with the healthy group were found. Further research aimed at assessing the clinical potential of such measurements is discussed, as are studies to elucidate the underlying mechanisms for the impedance changes. We propose the name 'dynamic electrical impedance myography' for this new technique.