Data on the pharmacokinetics of doxycycline in dogs and cats are reported. Doxycycline was given orally in the form of palatable tablets of Ronaxan. Tablets of 100 mg of doxycycline (as hyclate) were used in dogs, whereas cats were given tablets of 20 mg. The doses administered were 10 mg/kg in both species. The pharmacokinetics of doxycycline in dogs and cats were compared with those obtained in man (at a dosage of 3 mg/kg/day). It is concluded that a dosage of 10 mg/kg/day of doxycycline is required to obtain effective plasma concentrations in dogs and cats for 24 hours, when this dose is administered once daily.