Ruminants eat a variety of foods, varying in toxins and nutrients, but no hypotheses adequately explain this behavior. We offer an explanation, one which encompasses avoidance of toxins and acquisition of nutrients. A key concept in this hypothesis is aversion, the decrease in preference for food just eaten as a result of sensory input (taste, odor, texture, i.e., a food's flavor) and postingestive effects (of toxins and nutrients on chemo-, osmo-, and mechano-receptors) unique to each food. On the basis of this hypothesis, we predicted lambs would prefer familiar and novel foods that complemented the macronutrient composition of their basal diet. To assess the validity of this prediction, we fed 10 lambs in each of three treatments different levels of ground barley (high in energy) and alfalfa (high in protein) as a basal diet. We then offered them daily a meal of three ground foods differing in proportions of barley and alfalfa (familiar foods) or wheat and rabbit pellets (novel foods). We found that lambs fed a basal diet high in energy (barley) preferred food lower in energy and higher in protein (alfalfa); those fed a diet high in alfalfa preferred food high in barley (P < .01). In addition, the higher the barley or alfalfa content of the basal diet, the greater the acceptance of novel foods high in alfalfa (i.e., rabbit pellets) or grain (i.e., wheat), respectively (P < .01). All lambs preferred foods high in wheat to rabbit pellets or alfalfa (P < 0.01), evidently because wheat is high in energy and it differs in flavor from barley, which was eaten repeatedly as part of the basal diet. On the basis of these results, we contend that lambs preferred familiar and novel foods that complemented the flavors and macro-nutrient contents of their basal diet.