We have found that catecholamines stimulate DNA synthesis and centrosomal separation in 3T3 and bovine aortic endothelial cells cultured in the absence of serum or added growth factors. The mitogenic effect is mediated by an alpha 1-adrenergic receptor, as it is inhibited by phentolamine and prazosin but not by propranolol or yohimbine. The physiological and pathological consequences of this effect remain to be determined.