We have used high resolution magnetic resonance imaging to monitor malignant rat BT4C gliomas in vivo following herpes simplex virus thymidine kinase gene and ganciclovir (GCV) treatment. Twenty-six female BDIX rats were used for the study including four controls. Serial magnetic resonance imaging was performed every 72 hours to quantify tumor volume, transverse relaxation time (T2) ,and apparent diffusion constant (ADC) of water in the tumors and in the contralateral brain. GCV treatment was given twice a day, intraperitoneally, for 21 days. The gliomas exhibited low T2 and ADC values (before treatment), compared to normal brain, indicating the presence of high cell density tumors. Following GCV treatment, a regional increase in T2 and ADC was observed as early as day 4 of the treatment, even though the tumor volume was still increasing. These observations suggested evolution of local necroses which were confirmed by histology. In a group of five tumor bearing rats, retrovirus-producing packaging cell injections were given intratumorally to mimic clinically relevant gene therapy. In these cases, only small and short-lasting T2 and ADC elevations were found following GCV treatment without an effect on the overall tumor growth and outcome. Our results show that quantitative magnetic resonance imaging including T2 and ADC, is superior to robust volume measurements in predicting an early response to retrovirus-mediated gene therapy in vivo.