A high-performance liquid chromatographic procedure was developed for the determination of caffeine in various biologic fluids and coffee. A reversed-phase column and UV detection at 254 nm were used to obtain a sensitivity of 0.1 microgram/ml caffeine in serum and saliva using a sample volume of 0.1 ml. Caffeine metabolites and commonly ingested xanthines do not interfere with the assay. The within-day coefficients of variation were 9.8 and 9.9% at plasma caffeine concentrations of 2 and 10 micrograms/ml, respectively. The day-to-day coefficients of variation were 6.8 and 6.6% at plasma caffeine concentrations of 2 and 10 micrograms/ml, respectively. Serum and saliva caffeine concentrations were determined following a single oral dose of coffee and an intravenous infusion of caffeine in one subject. Computer estimates of caffeine pharmacokinetic parameters in one subject are in excellent agreement with previously published values.