Aneurysms were produced on the common carotid artery of rats by milliwatt CO2 laser welding of an adventitia patch over a hole. Sixty-nine animals were operated on. Aneurysms were present in 35 animals (51%), 7 of which ruptured spontaneously. The time to aneurysm formation was 1 week or longer. The adventitia patch aneurysm model has features (reliability, minimal vessel manipulation, histology similar to human berry aneurysm, and spontaneous bleeding tendency) that make it theoretically suitable for the induction of intracranial aneurysms in experimental animals. The microsurgical skills required to create and dissect the aneurysm make our experimental aneurysm useful as a teaching model in the development of microneurovascular surgery skills.