One thousand milligrams of aspirin, as its lysine salt was administered intravenously, orally, and intramuscularly to nine male and nine female young healthy adult volunteers. After intravenous injection mean (+/- SD) values of clearance, steady-state volume of distribution, and terminal half-life were 12.2 +/- 2.2 ml/min/kg, 0.219 +/- 0.042 liter/kg, and 15.4 +/- 2.5 min, respectively, with no differences between males and females. Following administration aspirin was absorbed more quickly in females than in males (mean absorption times of 16.4 and 21.3 min, respectively although the bioavailability, 54%, was the same in both groups. In contrast, following intramuscular administration, aspirin was absorbed more slowly in females than males (mean absorption time of 97 and 53 min, respectively) but again the bioavailability, 89%, was the same in both groups. The data suggest that in the female the intramuscular injection is going into fat. Salicylic acid concentration-time profiles showed a less pronounced sex difference and were comparable among the routes of administration.