The systemic availability of acetylsalicylic acid (ASA) after oral ingestion of 1 g in an effervescent formulation was 16.3 +/- 2.0% and 16.9 +/- 3.2% of the ingested dose in normal women and men, respectively. The average plasma half-life of ASA in each sex was also identical at 18.5 +/- 1.4 and 18.1 +/- 1.2 min, respectively. The inhibitory effect of ASA on collagen-induced platelet aggregation in vitro on blood from both sexes was studied. The IC50 was 23.9 +/- 2.9 micrograms/ml in females and 22.5 +/- 2.7 micrograms/ml in males, which did not differ significantly. The inhibition by salicylic acid (SA) of the antiaggregatory effect of ASA was similar in both sexes with increases in IC50 to 33.5 +/- 5.1 micrograms/ml in females (p less than 0.02) and to 29.5 +/- 3.8 micrograms/ml in males (p less than 0.05). It is concluded that the observed sex-difference in the antithrombotic effect of ASA cannot be explained neither by differences between females and males in the pharmacokinetic properties of ASA after oral ingestion, nor by differences in the in vitro effect of ASA on the platelet aggregation induced by collagen.