The potential use of PET to monitor radiotherapeutic effects on tumors has been evaluated with L-[1-11C]tyrosine and 18FDG. Single x-ray doses of 10, 30, or 50 Gy have been applied to rhabdomyosarcoma tumors growing in the flank of rats. Dose-dependent reductions of tracer uptake were registered by PET 4 and 12 days after treatment. These later effects on tracer uptake appeared to correlate with changes in tumor volume. Therefore, PET using L-[1-11C]tyrosine and 18FDG is suitable to monitor kinetics of tumor growth and tumor regression after radiotherapy. Direct effect on tracer uptake was not observed within 8 hr after irradiation. This indicates that, using PET, early predictions on the outcome of radiotherapy are not possible. When combining a radiation treatment with hyperthermia, radiation-induced inhibition of tumor growth was clearly enhanced. Tracer uptake remained at the pretreatment value, possibly due to invasion of host cells. From these experiments, it can be concluded that it is difficult to monitor a combined treatment of radiation and hyperthermia by PET.