Sympathetic nerves and catecholamines seem to have a trophic influence on vascular smooth muscle cells in vivo. We therefore tested whether alpha 1-antagonists would inhibit proliferation in arterial smooth muscle in vivo. Smooth muscle cells were stimulated to form a neointima in rat carotid arteries after deendothelialization by means of a 2F embolectomy catheter. The development of intimal lesions was determined 14 days after injury. The size of the neointima was measured using two parameters. Intimal DNA content was estimated from 5-mm segments of carotid arteries after DNA extraction and staining with Hoechst-stain. The size of the neointima was determined morphometrically as intimal area in histological cross-sections. Prazosin and urapidil were given orally once per day. Urapidil-treated rats showed significant inhibition of neointima formation for both parameters in a dose-dependent fashion. For prazosin a significant reduction could only be observed if DNA content was considered.