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1 A selection of novel compounds were shown to exhibit dopaminergic activity in vitro. 2 111 Indium-labelled platelets were continuously monitored in the cerebral and pulmonary vasculature of anaesthetized rabbits. The e ects of dopamine and selective dopamine receptor agonists on ADP and thrombin induced platelet accumulation were recorded. 3 Pretreatment with dopamine (2 mg kg 71 min 71 , i.v.) signi®cantly reduced ADP (20 mg kg 71 , i.v.) induced platelet accumulation in the pulmonary vasculature whereas lower doses had no e ect. 4 Dopamine (100 mg kg 71 min 71 intra-carotid, i.c.) potentiated thrombin (90 u kg 71 , i.c.) induced platelet accumulation in the cerebral vasculature whereas higher doses (1 ± 2 mg kg 71 min 71 ) inhibited accumulation. 5 The selective dopamine receptor agonists tested did not signi®cantly inhibit platelet accumulation induced by ADP or thrombin. Two of these selective agonists, at doses higher than the intended therapeutic doses, induced thrombocytopaenia and an associated increase in platelet accumulation in the lung in response to thrombin. 6 These results extend previous in vitro studies regarding the dual actions of dopamine upon platelets and show for the ®rst time the e ects of selective dopamine receptor agonists upon platelet aggregation in vivo.

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