In guinea pigs, sodium arachidonate-induced platelet aggregation ex vivo proved a more sensitive method for detecting drug-related effects after the administration of sulphinpyrazone than collagen-induced platelet aggregation. It was possible to detect inhibition of platelet aggregation after giving sulphinpyrazone orally in a single dose of 3 mg/kg. Excellent correlation was achieved between ex vivo and in vivo inhibition of platelet function with sulphinpyrazone using the Arthus reaction. The results are consistent with the formation, in the guinea pig of metabolites more potent than the parent molecule.