The relationship between tumour tissue oxygenation and necrosis at different tumour sizes was investigated in a C3H mammary carcinoma implanted in the feet of female CDF1 mice. Experiments were performed using tumours that ranged in size from 80 to 800 mm3. Necrosis was estimated histologically. Tumour tissue oxygenation (pO2) was estimated with an Eppendorf electrode. Our results showed that as tumour volume increased there was a corresponding increase in necrotic fraction ranging from 1% in small tumours up to 51% in large tumours. The percentage of pO2 values < or = 5 mmHg increased from 2% up to 79%, in small and large tumours respectively. After correcting for necrosis, the apparent, significant increase in the % of pO2 values < or = 5 mmHg was lost. We conclude that correcting for necrotic fraction in this tumour model is necessary when attempting to measure tumour oxygenation using electrodes.