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INTRODUCTION Volatile fatty acids (VFA) promote rumen development in calves (Sakata and Tamate, 1979) and rumen epithelial papilla growth in lambs (Lane et al., 1997). However, it was found that butyric acid and other VFA inhibited proliferation of rumen epithelium incubated in vitro (Kruth et al., 1982; Neogrady et al., 1989a; Baldwin, 1999). The inhibition effect of VFA, however, could be overcome by insulin-like growth factors -I (IGF-I), insulin and epidermal growth factor (EGF) (Baldwin, 1999). The difference of the VFA effect on rumen epithelium between in vivo and in vitro studies could be that VFA stimulated the secretion of IGF-I, GH and other hormones in vivo which in turn improved rumen epithelium growth, whereas in vitro, no hormones were secreted. In order to clarify this hypothesis, study of the quantitative relationship between VFA supply and IGF-I, IGFBP-3 and GH in ruminants is needed.