Previous studies have shown a good correlation between the degradation characteristics of roughages as measured by the in sacco nylon bag technique, and their voluntary intake by sheep (Hovell et al. 1986), and cattle (Wheeler et al. 1979; Ørskov et al. 1988). The greater intakes of better roughages are associated with a greater outflow of undegraded residues from the rumen. We have also found that even when the roughages were ground to a particle size small enough to leave the rumen without further comminution, the differences in intake between good and poor remained. Thus the differences in intake were not entirely due to material degraded in the rumen, but were associated with differences in the outflow of undegraded residues from the rumen, and therefore some animal factor appeared to be involved. The greater outflow of undegraded residues of the more degradable roughages appeared not to be mediated by the contractile behaviour of the reticulo-rumen (Hovell et al. 1987; 1988).