A series of investigations were undertaken in vitro and in vivo to investigate the nutritive value of cottonseed (WCS), the rumen microbial degradation of oilseeds and legume seeds, and the effects of gossypol on the rumen microbial (bacterial, protozoal and fungal) fermentation, and on the growth of animal cells in culture. An HPLC (High Performance Liquid Chromatography) method for the analysis of gossypol in WCS and in rumen liquor of sheep fed with WCS was also studied. Six oilseeds namely WCS (Gossypium sp.), mustard seed (Brassica sp.) (cv. Albatross), linseed (Linum sp.) (cv. Antares) and three cultivars of rape seeds (Brassica spp.) (cv. Bienvenu, cv. Libravo and cv. Samourai) were incubated with rumen contents in consecutive batch culture (CBC) and examined for their microbial dry matter (DM) degradation. For comparison, another selection of legume seeds namely narbon seed (Vicia narbonensis), mungbean seed (Phaseolus aureus) (cv. Satin), two cultivars of cowpeas (Vigna unguiculata) (cv. Banjo and cv. Caloona), and lablab seed (Lablab purpureus) (cv. Highworth) were also examined for their DM degradation. The oilseeds tended to show lower DM losses, gas production and volatile fatty acids (VFA) concentration than the legume seeds. It is uncertain whether the presence of fat, fractions within fibre such as xylose and lignin, and/or antinutrients such as gossypol in WCS and glucosinolates in rape seeds might contribute to the low fermentation. Although legume seeds are known to contain many antinutrients such as tannins, lectins, protease inhibitors and cyanogenic glycosides, they appeared more readily fermentable than the oilseeds. The overall results suggest that gossypol would affect the numbers of rumen protozoa and fungi, but gossypol was not the only factor and probably not even a major factor influencing the nutritive value of whole cottonseed. In particular much more detailed analysis on the plant cell wall structures such as the proportions of different sugars, and the amounts of lignin-carbohydrate complexes present would need to be performed to determine whether such factors might limit the degradability and fermentability of whole cottonseed. There is also possibility that antinutrients other than gossypol such as pigments tannins and flavanoids may also affect the rumen ecosystem.