Neonatal pig and rat aortas were studied for their ability to synthesize elastin in an in vitro situation. Smooth muscle cells from the rat aorta produced excellent multilayered cultures and produced soluble elastin (tropoelastin), insoluble elastin, and small amounts of collagen. BAPN proved to be toxic to these cells, adversely affecting the level of extracellular protein production. Tissue minces from pig aorta continued to synthesize elastin for two hours after removal. However, a 24 hour study indicated that elastin synthesis had almost completely shut down and that collagen synthesis continued in an apparently normal fashion. It is concluded that in vitro elastin synthesis is an extremely sensitive process easily altered by culture conditions and the addition of extraneous substances such as BAPN, and also highly influenced by the past history of the smooth muscle cells involved.