AbstractFollowing the demonstration of an antitremor effect of ethosuximide in the 1‐methyl‐4‐phenyl‐1,2,3,6‐tetrahydropyridine (MPTP) monkey model, we have tested the effect of this drug in 10 patients with typical parkinsonian tremor. Six patients suffered from Parkinson's disease with prominent, relatively drug‐resistant rest tremor. The other four patients had a druginduced parkinsonian tremor due to neuroleptic agents taken for a psychiatric condition. Five of the six parkinsonian patients and three of the four psychiatric patients reported within a few days a marked and intolerable exacerbation of their tremor. One patient in each group was improved, and this improvement was verified in repeated examinations. Thus, for unknown reasons, the effect of ethosuximide was not as predicted by the monkey model. Among possible explanations is the fact that we had chosen patients with drug‐res