(+)Fenfluramine decreases the 14C-5HT stored in rat platelets both in in vitro and in in vivo systems, indicating a release of the amine. The effect of (+) fenfluramine in vitro increases by increasing the concentration of the drug, the time of incubation with platelets and the temperature. It is not accompanied by loss of lactate dehydrogenase thus excluding an unspecific damage to the platelet membrane induced by the drug. In addition it is not inhibited either by inhibitors of the uptake of 5HT (chlorimipramine, Lilly 110140) or by inhibitors of the platelet 'release reaction' (acetylsalicylic acid) or by an excess of cold 5HT in the incubation medium. The effect of (+) fenfluramine in vivo is dose-dependent and increases gradually up at least 18 hr after i.p. drug's administration. It is significantly inhibited in rats pretreated by either chlorimipramine or Lilly 110140, but not by acetylsalicyclic acid. In analogy with the effect of (+) fenfluramine on rat brain 5HT, it is suggested that this drug could enter platelets by utilizing the 5HT uptake mechanism, at least in vivo.