The aim of the present study was to determine whether sympathectomy could influence the relative expression of two intermediate filament proteins, desmin and vimentin, two markers of differentiation, in arterial smooth muscle cells of the young rat. Newborn rats were treated with either repeated guanethidine or saline injections. Sections of the abdominal aorta, the femoral artery, the basilar and the middle cerebral arteries were processed simultaneously for immunofluorescence with monoclonal antibodies against desmin and vimentin and were then examined under either a conventional or a confocal laser-scanning microscope. In both cases, the mean optical density of the muscle layer staining was estimated by computerized image analysis. All artery sections from guanethidine-treated animals showed a significantly higher vimentin expression (108-119% of control values). Desmin expression was not significantly different except in the femoral artery after sympathectomy, where it was decreased by 6%. The relative increase of vimentin expression in sympathectomized blood vessels observed in the present study directly confirms previous morphometric and ultrastructural studies indicating that the sympathetic innervation of cerebral and peripheral blood vessels influences the phenotypic features of smooth muscle cells during post-natal development.