essential oils obtained by hydrodistillation of leaves from specimens of ageratum conyzoides and acritopappus confertus were investigated by gc-ms. the leaf essential oil of a. conyzoides consisted exclusively of the chromenes precocene i (95.4%) and ii (4.5%). the essential oil of a. confertus consisted predominantly of monoterpenes, representing 81.0% of the total. the main component was myrcene (52.0%), with reasonable amounts of -pinene (16.8%) and limonene (8.2%). the essential oil of a. conyzoides from the north-east of brazil has been reported as the major source of precocene i to date. copyright (c) 2005 john wiley & sons, ltd.