The mechanisms underlying clonus are considered in relation to current theories. By the use of a lever attached to a printed motor forces have been applied to the upper edge of the patella, a steady stretching force in suitable hemiplegic patients started the clonic oscillations. With rhythmic forces it has been found that the underlying rhythm is very resistant to entrainment. The oscillation provoked by a steady bias is increased in amplitude but unchanged in frequency when inertia is added to the apparatus. The frequency of clonus at the ankle, patella and wrist is similar. These and other considerations lead to the conclusion that clonus is not due to the self re-excitation of stretch reflexes but to an autonomous central generator.