The effects of verapamil on the metabolism of exogenous 14C-arachidonic acid were studied in human platelets in vitro. 1 mM verapamil decreased the formation of TXB 2 and HHT and increased that of PGE 2, PGD 2 and PGF 2α. The radioactivity at the area of 12-HPETE on the thin layer chromatography plate was also increased by 1 mM verapamil. In addition, 10 μM and 1 mM verapamil caused a slight decreasing trend in the amount of free unmetabolized arachidonic acid. The results suggest that high concentrations of verapamil may decrease the formation of the aggregatory thromboxane and increase that of anti-aggregatory compounds, i.e. PGD 2 and 12-HPETE in human platelets in vitro. However, as lower concentrations of verapamil (10 and 100 μM) had no significant effect on the metabolism of exogenous arachidonic acid, the anti-platelet effects of the drug at therapeutic concentrations are more likely to be mediated via other mechanisms, possibly via the inhibition of arachidonate release from the platelet membrane phospholipids.