Serial image-localized 31P magnetic resonance spectroscopy studies were performed in nine patients with newly diagnosed non-Hodgkin's lymphoma (NHL) during the early part of treatment with chemotherapy. The pre-treatment intracellular pH (pHi) of the tumours ranged from 6.97 to 7.61 for high-grade NHL (n = 3), and 7.16 to 7.39 for low-grade NHL (n = 5). A pH of 7.24 was recorded in a patient with intermediate-grade NHL. Slice-to-slice variation in tumour pHi in spectra obtained with a one-dimensional chemical shift imaging (1D-CSI) technique varied from zero to 0.5 pH units. The largest variation was seen in high-grade tumours. Slice-to-slice variation may reflect tumour heterogeneity. Alkaline shifts in tumour pHi of 0.14 to 0.45 pH units were seen in six patients following chemotherapy. Maximal change in tumour pH was related temporally to increases in the phosphodiester/beta-adenosine triphosphate ratio, and occurred before alterations in tumour size were documented. Cell death and necrosis may be associated with an alkaline shift in pHi due to cessation of H(+)-producing processes and release of basic components of proteins. An alkaline shift in tumour pHi may therefore be an early metabolic marker of response to chemotherapy.