The procyanidolic oligomers coming from Vitis vinifera L. (two fractions) and Cupressus sempervirens L. (three fractions), and the monomers, (+)-catechin, (-)-epicatechin were tested for their effects on angiotensin I converting enzyme (ACE) activity. The oligomers are the most active (dose '5U of 0.08 mg/mI for the fraction A of Vitis, the most active substance). Monomers have little activity. In vivo, the vasopressive response to Ang I is inhibited by approximately 20% to 40 % in the rabbit after administration of procyanidolic oligo