The objective of this study was to determine the influence of caffeine on aspirin bioavailability and pharmacokinetics in man. Two lots of the drugs were compared. The first lot (Lot # 1), which was caffeine-free, contained 650 mg aspirin plus 60 mg citric acid, whereas the second lot (Lot # 2) was caffeine-aspirin combination (650 mg aspirin plus 120 mg caffeine citrate, equivalent to 60 mg anhydrous caffeine). On two different occasions (2 weeks apart), the subjects received these two lots of drugs orally, i.e., each volunteer was given 650 mg aspirin (Lot # 1) or 650 mg aspirin plus 120 mg caffeine citrate (Lot # 2). It was found that caffeine significantly increased the rate of appearance as well as the maximum concentration of the salicylate in plasma by about 31 and 15 per cent, respectively. The area under the plasma concentration-time curve (AUC0 affinity) of salicylate was statistically higher in the subjects given the drug combination as compared to those given aspirin alone. Other pharmacokinetic parameters of the salicylate remained unchanged. It was therefore concluded that caffeine can increase the bioavailability of aspirin in man without any other effects on the salicylate disposition.