It was shown that the amplitude of the soleus Mmax and Hmax responses decreases in the course of long-lasting H-reflex studies. The peak-to-peak amplitudes of the Mmax and Hmax responses in the soleus muscle (and the Mmax in the tibialis anterior muscle and small hand muscles) were measured repeatedly for 1-3 h in 20 subjects. 3-5 Mmax responses and 5-10 Hmax responses were elicited about every 3 min while the subject was at rest. Decreases in the soleus Mmax response of up to 50.5% (mean 20.5% SEM 2.2) and of the soleus Hmax of up to 49.7% (mean 19.1% SEM 3.7) in relation to the amplitudes measured at the beginning of the experiment were seen in 17 subjects. In 3 subjects no Mmax amplitude decrease was seen. The maximum decrease was reached between 10 and 100 min (mean 44.2 min SEM 4.3). An Mmax amplitude decrease was also seen in the tibialis anterior muscle and in two small hand muscles. In some subjects the decrease of the Mmax response seemed to be initiated by the infrequent supramaximal stimulations. The possible causes for this amplitude reduction, as well as the methodological consequences of these findings for H-reflex studies and fatigue studies, are briefly discussed.