Studies on the action of 2-ethyl-3-(4-hydroxybenzoyl)-benzofuran (benzarone) on the metabolism of cultured arterial smooth muscle cells and arterial tissue had the following results: 1. On incubation of calf arterial tissue in the presence of 0.03--0.1 mmol/l benzarone (8--26 micrograms/ml medium) the metabolic transformation of [14C]-glucose to [14C]-lactate and 14CO2 and the incorporation of 14C radioactivity into the total lipids is not significantly altered as compared with control values. In cultured human arterial smooth muscle cells 0.03 mmol/l benzarone stimulates the incorporation of [14C]-acetate and [3H]-palmitate into the cellular lipids while the receptor mediated uptake of homologous low-density lipoproteins (LDL) by the cells and their release are not influenced. 2. In concentrations greater than 0.2 mmol/l benzarone the glucose utilisation of arterial tissue is enhanced, while the labelling of lipids, in particular the labelling of the triglyceride fraction, is depressed. Under the same conditions the protein biosynthesis and the incorporation of [14C]-acetate and [3H]-palmitate into the total lipids of cultured arterial smooth muscle cells are decreased.