Thalidomide has been shown to be an inhibitor of angiogenesis in a rabbit cornea micropocket model; however, it has failed to demonstrate this activity in other models. These results suggest that the anti-angiogenic effects of thalidomide may only be observed following metabolic activation of the compound. This activation process may be species specific, similar to the teratogenic properties associated with thalidomide. Using a rat aorta model and human aortic endothelial cells, we co-incubated thalidomide in the presence of either human, rabbit, or rat liver microsomes. These experiments demonstrated that thalidomide inhibited microvessel formation from rat aortas and slowed human aortic endothelial cell proliferation in the presence of human or rabbit microsomes, but not in the presence of rat microsomes. In the absence of microsomes, thalidomide had no effect on either microvessel formation or cell proliferation, thus demonstrating that a metabolite of thalidomide is responsible for its anti-angiogenic effects and that this metabolite can be formed in both humans and rabbits, but not in rodents.