The caffeine content of all tea or coffee beverages consumed by 17 healthy adults over 24 hours was measured. Plasma caffeine, theophylline, theobromine, and paraxanthine concentrations were determined over the same 24 hours. The average caffeine content per drink was 60.4 ± 21.8 mg for instant coffee (14‐fold range), 80.1 ± 19.2 mg for brewed coffee (2.8‐fold range), and 28.8 ± 13.7 mg for tea (5.5‐fold range). The number of drinks of coffee and tea consumed was a poor index of actual caffeine intake (r2= 0.42). Caffeine intake correlated poorly with the 24‐hour average caffeine concentration (r2= 0.41), but there was a very good correlation between a single plasma caffeine concentration measured at 5 PM and the 24‐hour average concentration (r2= 0.94). The same was true for paraxanthine (r2= 0.86). Paraxanthine accounted for 67.3% of the total dimethylxanthines in plasma, while theobromine and theophylline accounted for 24.4% and 8.3%, respectively. Mean caffeine clearance was 1.2 ± 0.3 ml/min/ kg. Plasma caffeine concentration before the first drink in the morning correlated very poorly with caffeine clearance (r2= 0.07), even when adjusted for caffeine intake (r2= 0.21).Clinical Pharmacology and Therapeutics(1986)39,54–59; doi:10.1