The purpose of the present study was to determine the relationship between biochemical parameters and the functional behaviour of an autonomously contractile and a non-contractile vascular smooth muscle (portal vein and vena cava inf. of the rabbit) in vitro. We studied the oxygen consumption and the utilisation of various substrates, the DNA content, the concentration of ATP, ADP and phosphocreatine, the extracellular space and the intracellular concentrations of Na⁺, K⁺ and Ca²⁺, the content of the iminoacid hydroxyproline and the microscopical characteristics of both vessels. The autonomously contractile portal vein differs definitely from the vessel without this functional speciality not only in regard to the fine structural and physiological characteristics but also to the metabolic behaviour and activity.