The definitive version is available at: http://dx.doi.org/10.1051/animres:2004001Previously published as proceeding at the XXXIII Jornadas de Estudio AIDA (IX Jornadas sobre Producción Animal) ITEA vol. Extra 22 (Zaragoza, Spain, Apr 25-27, 2001). https://digital.csic.es/handle/10261/13840This work was conducted to ensure that the consumption of a small amount of a chestnut hydrolysable tannin (HT) extract, included in the diet (20.8 g·kg–1 DM) of finishing lambs as a feed additive, does not cause toxic effects or does not negatively affect lamb performance. Thirteen Merino lambs were finished from 15 to 25 kg of live weight, which is the most typical slaughter weight for lambs in Spain. They were divided into two groups: one was used as the control (Control) and the other one received the treatment with tannins (TAN). The only difference between the groups was that the soya bean meal incorporated as the protein supplement in the TAN concentrate had been treated with the chestnut HT extract. No significant differences (P > 0.10) in voluntary intake, feed conversion, daily gain or length of fattening period were observed between the two groups. The histopathological examination showed no signs of toxicity due to the tannins. Likewise, the carcasses of the TAN group did not show residues of analysed HT metabolites (gallic acid, ellagic acid, resorcinol, pyrogallol and phloroglucinol). The raised activities of the enzymes GGT (gamma glutamyl-transferase) and AST (aspartate amino-transferase) in the TAN lambs suggest that experiments of longer duration need to be conducted, to further check for toxicity effects under these conditions.This work was supported by the Junta de Castilla y León (Spain, Project CSI 7/98).Peer reviewed