1. Levodopa, dopamine, noraderanaline and adrenaline (in increasing order of potency) depressed the tension and degree of fusion of incomplete tetanic contractions of the slow-contracting soleus muscle in chloralose-anaesthetized cats. 2. The effects of all compounds were antagonized by propranolol (50-20 microgram/kg), but not practolol (1.0-5.0 mg/kg). This indicates that effects are mediated by beta2-adrenoceptor stimulation. 3. The effect of levodopa, but not of the catecholamines, was antagonized by prior administration of the dopa decarboxylase inhibitior benserazide. This indicates that levodopa itself is inactive, whereas its decarbodylated metabolites are active. 4. The depressant action of beta-adrenoceptor agonists on incomplete tetanic contractions of the cat soleus muscle, which are exerted directly on the muscle fibres, is a model for effects exerted on slow-contracting units of human muscles; the latter effects probably underlie the tremor observed after beta-adrenoceptor agonist administration. 5. These results therefore suggest that levodopa, via its decarboxylated metabolites, dopamine, noradrenaline and adrenaline, may produce or exacerabate tremor in man. Thus in Parkinsonian patients any centrally induced relief of tremor that levodopa may produce may be masked by tremorogenic effects of such metabolites exerted in the periphery.