1. Relationships between the plasma and urine concentrations and clearances of caffeine over successive dosage intervals at steady-state were investigated in six healthy volunteers administered caffeine, 150 mg 8 hourly for 6 days. 2. There was marked inter-individual variability in the urine (15.9-fold range) and steady-state plasma (8.1-fold range) concentrations of caffeine. 3. Urine caffeine concentrations were similar to those in plasma, with mean ratios (plasma:urine) ranging from 1.10 to 1.74. There was a good correlation (r = 0.93, P less than 0.01) between caffeine urine and plasma concentrations. 4. There was a good correlation between caffeine renal clearance and urine flow rate (r = 0.89, P less than 0.01). Caffeine renal clearance was not significantly different from the product of fu and urine flow rate, where fu is the fraction of caffeine unbound in plasma. Urine caffeine concentration and urine flow rate were not correlated (r = 0.14, P greater than 0.05). 5. The results indicate that caffeine is reabsorbed from the renal tubule to equilibrium with unbound caffeine in plasma. 6. A regulatory urine caffeine concentration limit of 12 mg 1(-1) may be exceeded by some individuals with coffee intake in the range 3 to 6 cups per day.