Single voxel 1H localized spectroscopy (PRESS at 300 MHz) was used to monitor physiological and biochemical changes induced by hydralazine (5 mg/kg, i.p.) in murine C3H mammary tumours. In addition to a significant increase (by 52%, maximal at 30 min) in the intensity of the 1.32 ppm signal (predominantly from lactate, consistent with a selective reduction in tumour blood supply by hydralazine), downfield shifts in the resonance frequencies of 1H signals were observed. In particular, the signal initially at 3.24 ppm (total choline, tCho) shifted by 0.050 ppm (maximal at 13 min), whereas water shifted by 0.086 ppm. Lactate intensity and water and tCho resonance frequencies returned to control values at approximately 100 min after treatment. No significant changes in the resonance frequencies of water or tCho were observed over this time period in the tumours of mice given saline. In vitro studies showed that, while the resonance frequency of water was temperature dependent, the main components of the tCho signal (choline, phosphorylcholine and glycerophosphorylcholine) were more than 30-fold less sensitive to temperature. It was concluded that the shift in the water resonance frequency was due to the combined effects of tumour temperature reduction and a paramagnetic shift from increased deoxyhaemoglobin levels, whereas the tCho signal was only affected by the paramagnetic shifts.