Joint consideration of morphological studies, life-history data, and preferred food characteristics suggests that there may be optimal strategies for solid food supplementation during lactation for many mammals. This question was investigated by asking whether characteristics of food and morphological differentiation of the gastrointestinal tract as well as the specific differentiations of the weaning process in young eutherian mammals are related with each other and what such relationships might mean. Data on body mass, food quality, the differentiations of the digestive tract, and length of lactation and the weaning period represented the basis of the following discussion. A relatively long period when milk is supplemented by solid food is advantageous for the mother because she does not have to supply the total caloric needs of the young during lactation. On the other hand, an extended absolute length of the mixed-feeding period is advantageous for the offspring because energy is supplied by the solid food and supplemented by milk. Animals that eat high-quality food are characterized by a relatively short mixed-feeding or weaning period. In Eutheria that eat a food rich in plant cell wall material, the digestive tract shows high complexity and more than 40% of the lactation period is characterized by mixed feeding. The mother tries to reduce her energy expenditure as much as possible, while the offspring tends to obtain as much energy and building material for its developing body as possible. Both mother and young try to optimize their specific energy situation.