BACKGROUND Poor tumour oxygenation is associated with radiation resistance. The recording of pre-treatment oxygenation status has been shown to be of prognostic relevance. MATERIAL AND METHODS Eleven dogs with spontaneously arising soft tissue sarcomas were included in this study. Oxygen partial pressure measurements (pO2) were performed with the Eppendorf method. RESULTS The mean of median pO2 was 9.6 mmHg (range: 0.1-30 mmHg). Four of the nine dogs included in the statistical analysis showed a median pO2 < or = 2.5 mmHg. The natural logarithm of the hypoxic subvolume correlated with the hypoxic fraction < or = 2.5 mmHg (p = 0.0712) and < or 5 mmHg (p = 0.0988). Mean corpuscular hemoglobin (MCH) and mean corpuscular volume (MCV) correlated significantly to several oxygen parameters. CONCLUSION Hypoxia exists in spontaneous canine soft tissue sarcomas and the dog can be used as a reliable model for repeated oxygenation measurements. Ultrasonography assures reliability of needle placement.