Seventy parkinsonian patients (mean age: 59.6 +/- 1.2 years; duration of disease: 9 +/- 0.6 years) with severe fluctuations of disability under L-Dopa treatment received a single dose of L-Dopa (200 mg + benserazide, a peripheral decarboxylase inhibitor) after 24-72 h interruption of treatment. The delay and duration of action of a single dose of L-Dopa, and the percentage of improvement of the parkinsonian symptoms were 39 +/- 2 and 162 +/- 6 minutes, and 57 +/- 2 p. 100 respectively. Estimation of the difference between the basal parkinsonian score and the score during maximum clinical improvement under levodopa treatment, and the score under levodopa treatment may reflect the severity of dopaminergic and of non dopaminergic lesions in the brain respectively. Modification of the treatment to obtain continuous clinical improvement can be performed according to the delay and duration of action of a single dose of L-Dopa.