An experiment was conducted with eighteen rumen cannulated sheep fed on a chopped lucerne diet. Previous level of intake significantly influenced the level at which sheep initially established voluntary feed intake. This difference had disappeared after three weeks on an ad lib. intake. Perturbation analysis of the results of the chopped lucerne diet and a high concentrate diet showed that sheep at low intakes, accommodated increased intakes mainly by increasing the rate constant which describes the fermentation and outflow of fermentable OM. This rate constant became less important as intake was increased and was progressively replaced by rumen fill, which accommodated 65% of increased intake at 1400 g OM intake.