Aspirin, 250 mg/day, was administered to 14 normolipidemic healthy subjects for 7 days. Platelet lipid composition was determined in washed platelets by quantitating cholesterol to phospholipid molar ratio. Platelet membrane fluidity was measured by steady state fluorescence polarization using the probe diphenyl hexatriene. Upon 7 days aspirin ingestion platelet lipid composition was not altered. There was a sex-dependent effect of aspirin administration on platelet membrane fluidity. Whereas overall platelet membrane fluidity did not change at 37°C, there was a significant decrease in female subjects; the anisotropy parameter which is inversely related to membrane fluidity increased from 0.937±0.043–1.048±0.027 (p<0.01). In male subjects there was an increase in platelet membrane fluidity, which was significant at 25°C; the anisotropy parameter decreased from 1.350±0.039–1.283±0.023 (p<0.05). These results indicate that aspirin alters the membrane dynamics of platelets. This effect results from mechanisms other than alterations in platelet cholesterol or phospholipid content and operates in an opposite direction in men and women.