It was hypothesised that both inadequate and abundant levels of effective rumen degradable protein (eRDP) would be avoided by ruminants given a choice of foods (Tolkamp et al, 1998). However, we have found that sheep do not appear to avoid the excess consumption of eRDP expected when they were offered a choice of foods that differed in eRDP content through the addition of urea to one of the foods (James et al., 2001). A possible reason could be due to the fact that the foods offered had low fibre contents. The sheep may have selected for the urea supplemented food in an attempt to buffer the rumen. The buffering ability of the urea could help to minimise the disruption of the rumen such as a fall in pH caused by the low fibre foods. The objective of this experiment was to test the hypothesis that the preference for the urea supplemented food would be reduced by the addition of sodium bicarbonate (buffer).