[en] Purpose: Only few quantitative data are available on late effects in the healthy brain after radiosurgery. An animal model would contribute to systematically investigate such late effects. For this reason a rat model applying radiosurgery at the rat brain was established. This investigation comprised several steps: (1) design for a special fixation- and localization device to perform linac based stereotactic radiosurgery at the rat; (2) feasibility and accuracy study of irradiation and MRI-evaluation in the rat brain; (3) a long term (1 (1(2)) year) follow up study with a group of animals. Materials and Methods: A localization technique was developed to irradiate a small target volume within the rat brain. A mean spatial uncertainty of 1 mm was verified by phantom measurements. At 60 animals, a small area of the brain was irradiated stereotactically (15 MV linac). Different doses doses of 20, 30, 40, 50, and 100 Gy with two field sizes using the 2 and 3 mm collimator were administered. These dose levels were selected to be equally distributed between 0 and 100% effect probability according to the Flickinger model assuming comparable radiosensitivity between rat and human brain. The diameter of the spherical dose distribution (80%-isodose) was 3.9 and 5.8 mm, respectively. The alteration of the permeability of the blood brain barrier was investigated, using magnetic resonance imaging and Gd-DTPA contrast agent. An intracranial contrast enhancement was interpreted as a first indication for brain necrosis. Half of the animals were killed for histology after 9 and 18 months, respectively. Results: A first intracranial signal enhancement was observed 160 days after irradiation. Within one year, all animal in the two 100 Gy groups showed contrast enhancement, but none of the other groups. The incident rates were (6(6)) for the 2 mm collimator and(5(5)) for the 3 mm collimator. Contrast enhancement volume and signal intensity were significantly different between these two groups. After 18 months, however, other animals also showed contrast enhancements. The incidence rate was (2(3)) for the 50 (Gy(3)) mm group and the 40(Gy(3)) mm group, and (1(3)) for the 30 (Gy(3)) mm group and the 50 (Gy(2)) mm group. All other animals did not show any contrast enhancement within 18 months. Conclusions: Linac based stereotactic radiosurgery can be successfully applied at the rat brain. The animal model is appropriate to study late normal brain tissue response. Contrast enhancement as an indication of late radionecrosis was found 1 (1(2)) year after even moderate doses of radiosurgery. The behaviour of radioresponse appeared to followed the prediction of the Flickinger model for human brain. The techniques used can also be applied to study modifications in the irradiation modality, i.e. fractionation, irregular volumes, or radiation quality