We evaluated the use of hyperglycaemia to reduce tumour pH in dog with spontaneous tumours. Dogs were randomized to two groups: control and glucose. Intravenous administration of 20% glucose was used to induce and maintain hyperglycaemia. Extra- and intracellular tumour pH were measured using interstitial pH microelectrodes and 31P-MRS, respectively. During the administration of glucose, the mean (+/- SEM) blood glucose concentration was 419.8 (+/- 32.8) and 121.1 (+/- 8.0) mg/dl for the glucose and control groups, respectively. The mean extracellular tumour pH before and following 90 min of hyperglycaemia was 7.15 (+/- 0.08) and 7.15 (+/- 0.09). During consecutive measurements, intracellular tumour pH did not change significantly for the control group or the group subjected to hyperglycaemic manipulation. In contradistinction to previous rodent studies, our results demonstrate that hyperglycaemia alone is not sufficient to manipulate either intra- (pHi) or extracellular (pHe) hydrogen ion concentration in spontaneous canine soft tissue tumours.