SummaryIn contrast to hypertonic urea which causes an immediate but transient increase in absorbancy of human platelet‐containing plasma, hypertonic antipyrine causes an immediate reduction of absorbancy.The urea effect is characteristic for human and pig platelets; antipyrine affects both human and dog platelets.Antipyrine in isotonic concentration causes little change in absorbancy of platelet‐containing plasma but platelet aggregation by adenosine diphosphate is inhibited at pH values between 7·1 to 7·9.As inhibitors of platelet adenosine diphosphate aggregation, butazolidine, tanderil and anturan are more potent than antipyrine.