The concentrations of doxycycline in serum and in thoracic duct lymph at various times after oral and intravenous administration of 200 mg of the drug were determined in 10 patients subjected to thoracic duct cannulation for diagnostic purposes. After oral administration, a mean peak serum concentration of 2.4 μg/ml (n = 7) was obtained within 3 h; then the levels successively declined. The concentrations in thoracic lymph were lower, a mean peak concentration of 1.6 μg/ml being found 3 and 6 h after the intake. After intravenous administration (n = 3), the concentrations of doxycycline in thoracic duct lymph were lower than but closely followed those in serum. The difference in concentrations between serum and lymph 1–6 h after the start of the infusion never exceeded 30%. The results suggest that doxycycline is rapidly distributed to the extracellular tissue flu