1. To assess the role of descending monoaminergic pathways for motor activity long-lasting EMG recordings were performed from the adult soleus muscle before and after selective depletion of spinal monoamines. 2. Rats were chronically implanted with an intrathecal catheter placed in the lumbar subarachnoid space and gross-EMG recording electrodes in the soleus muscle. EMG recordings were performed in control conditions and at different times after intrathecal administration of either 40-55 μg 5,6-dihydroxytryptamine (5,6-DHT) and 40-55 μg 6-hydroxydopamine (6-OHDA) or 80 μg 5,7-dihydroxytryptamine (5,7-DHT) alone. The depletions were evaluated biochemically in brains and spinal cords after recordings. 3. In agreement with previous studies the intrathecal administration of neurotoxins caused a reduction of the noradrenaline (NA) and serotonin (5-HT) content of the lumbar spinal cord to about 2-3% of control, with little or no changes in the monoamine content of the cortex. 4. In non-treated chronically catheterized rats the integrated rectified gross EMG displayed long-lasting EMG episodes composed of phasic high-amplitude events and tonic segments of varying duration and amplitude. 5. After intrathecal administration of neurotoxins the number of long-lasting gross-EMG episodes, the mean episode duration, and the total EMG activity per 24 h, were reduced. These changes were accompanied by a simultaneous increase both in the number of short-lasting EMG episodes and the total number of EMG episodes per 24 h period. The changes were apparent 5-6 days after drug administration and fully developed after 2-3 weeks. 6. No changes in general movement ability were observed, except that the denervated animals had a tendency to a less erect posture. 7. These results indicate that descending monoaminergic pathways are important for the maintained motor output in tonic hindlimb muscles.