The effect of buflomedil (4-(1-pyrrolidinyl)-1-(2,4,6-trimethoxyphenyl)-1-butanone) in vitro and ex vivo after intravenous and oral administration was tested upon epinephrine-enhanced platelet aggregation (PA) in platelet-rich plasma (PRP) prepared from heparinized blood of healthy volunteers. In vitro incubation of PRP with buflomedil in concentrations above 10 mumol/l resulted in a significant depression of PA to approximately one-third of the control. 30 min after a single intravenous dose of 2.5 mg/kg buflomedil a depression of epinephrine-enhanced PA to about 60% of the value before injection of the drug was observed. This effect wore off during a few hours and was no longer present 24 h thereafter. Oral ingestion of 600 mg/d buflomedil depressed PA to approximately two-thirds within 2 days, with a further decrease to some 50% after 6 days of intake. 2 days after termination of treatment epinephrine-enhanced PA had returned to premedication values. Unlike nonsteroidal antiinflammatory drugs buflomedil does not act through an inhibition of prostaglandin synthesis.