[en] This study was designed to assess the potential of Tl-201 scintigraphy in determining the therapeutic effects. Doses of 20 and 40 Gy were delivered to VX-2 tumors implanted in rabbits. Tumor Tl-uptake was quantitatively evaluated by determining the count ratio of Tl uptake in the tumor to that in the contralateral normal tissue. Tumor shrinkage or disappearance was seen in the 40 Gy group. In the 20 Gy group, however, some tumors were shrinked or disappeared, but the others were enlarged. In the 20 Gy group of tumor shrinkage or disappearance, the Tl-uptake ratio 7 days after irradiation was significantly lowered, as compared with that in the group of tumor enlargement and the non-irradiated group; and after that it was even more lowered. In the 20 Gy group, tumor volume was also slightly larger 7 days after irradiation than that before irradiation. In comparing the group in which Tl-uptake ratio decreased by 30% or more and the group in which it decreased by less than 30% after irradiation, tumor volume was inhibited or disappeared in the former but was increased in the latter. The uptake of BrdU, which is thought to reflect tumor proliferative potentiality, was extremely decreased associated with decreased Tl-uptake ratio in the 40 Gy group in which tumor volume was only inhibited 7 days after irradiation. Tl-201 scintigraphy may have a potential in evaluating tumor proliferation relatively early after radiation therapy, irrspecstive of tumor volume. (N.K.)