A theory of diet selection (Kyriazakis, 1989), which is supported by the results of a series of experiments, proposes that growing pigs, given a choice between a low and a high protein food, will select a diet that meets their protein requirements and avoids, at least to a certain extent, excess protein intake. It is important to test whether the rules that appear to underlie the diet selection of pigs persist when one, or both, of the foods given as a choice contain an anti-nutritive factor. A rapeseed meal of high glucosinolate content, was chosen as the protein source to test the above hypothesis. The objective of the experiment wad thus to investigate the rules that may underlie the diet selection of pigs when they are given a choice between two foods that differ in their protein content (a low and a high) and the inclusion level of rapeseed meal.