The use of antimicrobial growth promoters in European poultry production will be banned in 2006, which has made it economically viable to investigate the use of other acceptable compounds, such as botanical products. Botanical products have a wide range of known pharmacological effects which could be exploited, for example carvacrol, found in oregano, has antibacterial properties. Six experiments investigated the effects of botanical products on broiler growth performance. Two experiments were carried out to identify potentially useful products from a choice of six (garlic - Allium sativum, horseradish - Amoracia rusticana, juniper - Juniper communis, milk thistle - Silybum marianum, oregano - Origanum vulgare and yarrow - Achillea millefolium). Garlic powder and yarrow supplementation improved FeE (PO.05). Feeding yarrow supplemented diets resulted in improved weight gain (PO.05). Yarrow contains 'bitter' sesquiterpene compounds (cadinene and germacrene), which may stimulate digestive enzyme production, so the fifth experiment examined the effects of dietary yarrow on digestive enzyme activity and nutrient availability of birds fed control and low nutrient density basal diets. Yarrow supplementation increased intestinal lipase activity and improved diet AME in birds fed high nutrient density diets but not in birds fed low nutrient density diets (yarrow x diet density interaction P < O.05). Therefore, the final experiment examined the interactions between yarrow supplementation and fat source. Yarrow supplementation improved growth performance (P < O.Ol) of birds fed saturated fat sources with a concomitant increase in gizzard bile acid concentrations (P