In the Parkinsonian syndrome tremor is the symptom which still eludes pharmacological therapy. The authors report the results of nine cases of Parkinsonian tremor in which agonist and antagonist drugs acting on the receptors were employed. These were dopaminergics, acetylcholinergics, serotoninergics, histaminics, and alpha- and beta-adrenergics. The data obtained demonstrate the involvement of all the neurotransmitters studied, some directly through quantitative modifications, others indirectly through their involvement in reciprocal functional equilibria. A sytem of equilibria is hypothesized in which the histamine: serotonin ratio is given greater emphasis. (Acta neurol belg., 1977, 77, 213-229).