A new ent-kaurane diterpenoid, together with eight known ent-kauranes, were isolated from the petroleum ether and acetone extracts of the whole plant of Sideritis congesta P. H. Davis & Hub.-Mor. and their structures were elucidated as the new compound ent-7 alpha-acetoxy-16 beta, 18-dihydroxy-kaurane (7-acetyldistanol) (1) and the known compounds ent-3 beta,7 alpha-dihydroxy, 18-acetoxy-15 beta,16 beta-epoxykaurane (epoxyisolinearol) (2), sideroxol (3), sideridiol (4), siderol (5), 7-epicandicandiol (6), foliol (7), linearol (8) and sidol (9). Characterization of compounds 1-9 was based on spectral analyses and comparison with reported data, particularly the new compound 1 was identified by 1D- and 2D-NMR and mass spectroscopic analyses. The antioxidant potential of the extracts, and also of the ent-kauranes except for 7, was investigated by three methods including beta-carotene bleaching method, free radical scavenging activity and superoxide anion scavenging activity. The anticholinesterase activity was also evaluated for the ent-kauranes except for 7, and most of the diterpenes exhibited weak acetylcholinesterase inhibitory activity. However, almost all diterpenes exhibited some inhibitory activity against butyrylcholinesterase; particularly, compounds 3 and 6 exhibited better BChE inhibitory activity than the standard compound galanthamine. (C) 2011 Phytochemical Society of Europe. Published by Elsevier B. V. All rights reserved.