Increased capacity for glycolytic metabolism is a well-known characteristic of neoplastic cells. Because lactic acid is the end product of glycolysis, in vivo MRS measurements of tumor lactate concentration ([lac]t) may provide valuable information about tumor metabolism, which will aid the development of therapies and the clinical diagnosis and treatment of tumors. In the present study, several hemodynamic and histologic parameters were evaluated with respect to their influence on [lac]t. Pronounced differences in [lac]t in two distinct populations of tumors suggested a putative perfusion threshold. Above this threshold, [lac]t was independent of hemodynamic and histologic factors including tumor blood flow (measured using MRS and the method of D2O washout), extent of necrosis and inflammatory cell infiltrate. Thus, for most tumors, [lac]t was not determined by any one single factor such as hypoxia, venous clearance, glucose supply, extent of necrosis or degree of inflammatory cell infiltrate. Rather, [lac]t may be equilibrated, at least in part, by an interplay of forces involving hemodynamics and substrate supply. In general, the data are consistent with the hypothesis that elevated lactate in most tumors is related to the high glycolytic activity of adequately perfused, viable neoplastic cells.