The aim of this study was to test the reliability of the commonly used blocking technique for measuring the conduction velocity spectrum of the peripheral motor nerve in man, as first described by Hopf in 1962. Electromyographic recordings were carried out with a selectivity permitting identification of single motor unit potentials. The conduction velocity of the single alpha motor axon was determined by a direct technique and by the blocking technique. the refractory period was determined by comparison of the results of these two techniques. The refractory period was studied in relation to the strength of the testing stimulus, axonal conduction velocity, skin temperature and age of the subject. When the strength of the test stimulus increased from 10% to 50% above the axonal threshold, the refractory period decreased from 1.7 to 0.6 ms. An inverse relationship between the axonal conduction velocity and the refractory period was observed. When skin temperature was 4 degrees C subnormal the refractory period was significantly prolonged; at 10 degrees C subnormal it was more than doubled. The same relationship between axonal conduction velocity and the refractory period was observed in both young and elderly subjects. The significance of these findings for the clinical application of the blocking technique is discussed.