IN anaesthetized rats the influence of leukotriene (LT) D, on the discharges of group III and IV muscle receptors was studied. The impulse activity of single slowly conducting afferent fibres from the gastrocnemius-soleus muscle was recorded and their responsiveness to mechanical stimulation of the muscle determined. Intramuscular injections of LTD, 100 ng or 1 μg were made into the receptive field of units in intact and carrageenan-inflamed muscle in order to find out whether LTD, has a sensitizing action on the receptors. The LT reduced the mechanically induced responses in intact but not in inflamed muscle, whereas the background activity was not influenced in intact muscle and showed a decrease in inflamed muscle. The results demonstrate that LTD, does not belong to the substances which sensitize nociceptors and cause subjective tenderness following a tissue lesion.