The use of biodegradable polymers in animal nutrition can be considered not only as a possible way of waste recycling but could also be beneficial in the derivation of animal products from the conversion of these energy-rich substances within the nutritional chain. However, earlier studies with swine and sheep as well as in vitro studies have shown a poor digestibility of untreated biodegradable plastics (Kunkel and Seo 1994; Forni et al. 1999a,b). The low accessibility of the polymeric chains, particularly if only coarsely milled, to the gastric juices, enzymes and gastrointestinal micro-organisms of farm animals, and the relatively short incubation time during the passage through the digestive tract obviously limit the degradation of these biodegradable polymers. Therefore, the present study focused on the effects of pretreatment and of increasing the dietary proportion of the biodegradable plastic poly(3-hydroxybutyrate-co-3-hydroxyvalerate) (PHBV). PHBV is an aliphatic biodegradable polymer produced by bacteria (de Koning et al. 1996). Pretreatment with NaOH was chosen to reduce the chain length and to form absorbable monomers such as β-hydroxybutyrate, a natural metabolite in the digestive turnover and metabolism of ruminants. NaOH is an inexpensive and effective substance for breaking down PHBV, which was found to increase the apparent PHBV digestibility by 37 % in swine (Forni et al. 1999a). In addition, NaOH pretreatment is commonly used in ruminant nutrition to enhance the digestibility and intake of straw and poor quality hay which are both rich in biological polymers such as cellulose and lignin (Flachowsky 1987).