ABSTRACT — As we have earlier shown, sudden extreme changes in disease severity are a characteristic feature of Parkinson's disease. These variations take two main forms, those natural to the disease and those occurring as a result of treatment. Treatment-related response variations can sometimes be attributed to the known pharmacokinetic properties of anti-parkinsonian drugs and, in particular, levodopa. These responses are time-dependent. In contrast, other types of response variation may be unpredictable in time, and are at present rarely amenable to treatment. Identification of the cause of variation is necessary for the management of parkinsonism, and particularly for the successful use of deprenyi.