Background: Heparin treatment has been recommended for dogs in hypercoagulable states such as disseminated intravascular coagulation, however, potential benefits have to be balanced against the bleeding risk if overdosage occurs. A better understanding of the pharmacology of heparin and tests to monitor heparin therapy in dogs may help prevent therapeutic hazards. Objectives: The purpose of this study was to evaluate the effects of 200 U/kg of sodium unfractionated heparin (UFH) on coagulation times in dogs after intravenous (IV) and subcutaneous (SC) administration and to compare these effects with plasma heparin concentrations assessed by its antifactor Xa (aXa) activity. Methods: 200 U/kg of UFH were administered IV and SC to 5 healthy adult Beagle dogs with a washout period of at least 3 days. Activated partial thromboplastin time (APTT), prothrombin time (PT), and plasma aXa activity were determined in serial blood samples. Results: After IV injection, PT remained unchanged except for a slight increase in 1 dog; APTT was not measurable (>60 seconds) for 45–90 minutes, and then decreased gradually to baseline values between 150 and 240 minutes. High plasma heparin concentrations were observed (maximal concentration = 4.64 ± 1.4 aXa U/mL) and decreased according to a slightly concave-convex pattern on a semilogarithmic curve, but returned to baseline slightly more slowly (t240–t300 minutes) than did APTT. After SC administration, APTT was moderately prolonged (by a ratio of 1.55 ± 0.28 APTT t0, range 1.35–2.01) between 1 and 4 hours after administration. Plasma aXa activity reached a maximum of 0.56 ± 0.20 aXa U/mL (range 0.42–0.9 U/mL) after 132 ± 26.8 minutes; this lasted for 102 ± 26.8 minutes. Prolongation of APTTs of 120–160% corresponded to plasma heparin concentrations of 0.3–0.7 aXa U/mL. Conclusions: As in humans, the pharmacokinetics of UFH in dogs was nonlinear. Administration of 200 U/kg of UFH SC in healthy dogs resulted in sustained plasma heparin concentrations in accordance with human recommendations for thrombosis treatment or prevention, without excessively increased bleeding risks. In these conditions, APTT can be used as a surrogate to assess plasma heparin concentrations. These findings need to be confirmed in diseased animals.