The basis of diet selection for protein by ruminants has been questioned by Tolkamp et al. (1998). They suggested that diet selection by ruminants is based on the ruminally degradable protein (RDP) of foods whereas the metabolizable protein (MP) yield has no effect when the foods offered as a choice contain adequate RDP. However, it is necessary for an animal to have some knowledge of the nutritional properties of the available foods in order to select appropriately between them, as the prevailing view is that its feeding behaviour depends largely on learning (Arsenos and Kyriazakis, 1999). The objective of this study was threefold: (i) to test whether sheep are able to form specific associations between food flavours and post-ingestive consequences (PIC) induced by the administration of a ruminally undegradable, but readily digestible protein source (DUP); (ii) to investigate the relative importance of RDP and DUP sources and their consequent PIC in the development of such associations; and (iii) to test whether such associations can be formed when DUP is administered concurrently with a RDP source. Following Tolkamp et al.'s (1998) suggestion we have hypothesised that learned associations would be dictated by the PIC attributed to RDP rather than DUP administration.