This investigation estimated the mechanisms of baclofen action on spasticity using a battery of electromyographic methods. Thirty patients with old post-stroke spastic hemiparesis took part in the investigation. They were treated with baclofen-mean daily dose 54.3 alpha 11.6 mg for a mean of 26.3 alpha 4.9 days. A questionnaire for assessment of subjective improvement after treatment used a 5-point scale. For standardization of the neurological examination 5-point scales were used to assess muscle tone, muscle force and tendon reflexes. A battery of electromyographic methods was used to analyse different mechanisms of spasticity: for alpha motoneurone activity--the F wave parameters; for gamma motoneurone activity--the T/H reflex amplitude ratio; for presynaptic inhibition--the ratio of H reflex amplitudes before and after vibration on the achilles tendon (Hvibr./Hmax); for common interneurone activity--the flexor reflex parameters. Our results revealed that baclofen reduces spastically increased muscle tone and Babinski sign. It has no influence on muscle force, tendon reflexes and ankle clonus. Baclofen acts by normalizing the altered interneurone activity and decreasing of alpha motoneurone activity. When spasticity has altered interneurone activity and increased motoneurone activity, it is better to treat with baclofen.