Attempts are being made to evaluate 2-[18F]fluoro-2-deoxy-D-glucose (FDG) as a noninvasive marker of therapy response in malignant tumors. We studied rats with 7,12-dimethylbenzanthracene (DMBA)-induced mammary carcinomas by measuring the differential absorption ratio (DAR) and the metabolites of FDG in tumor homogenates. Half the rats were treated with the antiestrogen toremifene for 14 days and half were untreated. The histology of the tumors was studied by morphometry. The animals were killed 15, 45 or 240 min after injection. Regardless of whether the rats received toremifene or not, the fractional change in tumor volume correlated better with the DAR at 15 min [r = 0.284 (untreated) and r = 0.721 (treated)] and at 240 min [r = 0.932 (untreated)], than at 45 min [r = -0.137 (untreated) and r = 0.265 (treated)]. Inverse relations were found for the fraction of unmetabolized FDG and change in tumor volume [r = 0.070 (45 min) and r = -0.872 (240 min) for untreated tumors and r = -0.963 (15 min) and r = -0.715 (45 min) for treated tumors]. The DAR and the fraction of unmetabolized FDG correlated also [r = -0.420 (15 min), r = -0.647 (45 min) and r = -0.976 (240 min) for untreated tumors, and r = -0.963 (15 min) and r = -0.213 (45 min) for treated tumors]. No significant therapy-induced morphometrical changes were observed.(ABSTRACT TRUNCATED AT 250 WORDS)