The biosynthesis of squalene has been demonstrated in peas (Pisum sativum) in two different ways. The first of these involved germination of the seeds for a 5-day period in the presence of a large amount of mevalonic acid. The squalene, which was produced in a 40% yield, was identified by several physical and chemical properties. The second way in which squalene was experimentally biosynthesized was through a very short (24 hours) germination in the presence of a small quantity of 2-Cl4-mevalonic acid. The radioactive hydrocarbon was produced in a 38% yield and was identified by conversion to the hexahydrochloride and co-crystallization with a standard sample until constant specific activity was reached. Furthermore, the radioactive squalene was incubated with a rat liver homogenate, and radioactive cholesterol (purified through the dibromide) was produced in a 7% yield. This work represents the first experimentally achieved biosynthesis of squalene in a flowering plant, and it gives credence to the previous assumption that the β-amyrin and β-sitosterol biosynthesized in peas arise by hydroxylative cyclization of this hydrocarbon. Evidence is presented which indicates that isopentenoid biosynthesis is regulated physiologically by the control of mevalonic acid formation.