The effects on fusimotor discharge rate of algesic agents (bradykinin, potassium chloride, histamine, 5-hydroxytryptamine) and lactic acid, applied by close arterial injection into triceps surae muscles, were investigated in decerebrate cats. Fusimotor discharge was recorded from filaments dissected free from otherwise intact nerves to the triceps muscles. The substances applied induced an increase in discharge rate of spontaneously active gamma fusimotor neurones as well as a recruitment of previously silent ones. Skeletomotor discharges and/or muscle tension changes occurred only occasionally. The increase in fusimotor discharge rate was not always completely abolished by severing the nerves to triceps. What remained was a short-lasting burst at the very onset of blood-pressure fall. It was concluded that the increase in fusimotor discharge rate was mainly, but not solely, elicited reflexly by discharges from Group III and/or IV muscle afferents sensitive to algesic agents. An elevated fusimotor activity might be expected to accompany muscle inflammation and/or trauma when these agents are liberated in muscle tissue. The increase in fusimotor discharge rate elicited by lactic acid injections indicates that the fusimotor system might also be involved in neural processes of muscular fatigue.