Doxycycline levels in various organs of mice after i.v. and p.o. administration were measured by a high-performance liquid chromatography method. When given as a single dose (50 mg/kg) i.v. doxycycline accumulated in the lung. High concentrations were also measured in liver and kidneys. When 50 mg/kg doxycycline were administered by stomach tube to fed mice the antibiotic accumulated to a lesser degree in the lung. On the other hand, the antibiotic levels in the liver and the kidneys were higher as after i.v. injection. The absorption of doxycycline administered p.o. was about 92% and seemed not to be influenced by food in the stomach. The elimination half-life in mice was 170 min independent of the route of administration.