Two feeding trials on hand-reared grey partridges Perdix perdix were performed to study the effect of a change from a commercial to a natural diet on body mass, food consumption, metabolised energy coefficient, gut morphology and some blood metabolites. We simulated the abrupt change in the diet which takes place when hand-reared birds are released into the wild. In the test group body mass decreased significantly after the change in diet. However, within one week body mass started to increase again, but it stabilised at a lower level than in control birds. Birds in the test group consumed more food (fresh weight) during the feeding trial and even produced more excreta during the second, fourth and fifth week of the feeding trial. Gross energy intake, amount of metabolised energy and metabolised energy coefficient decreased and excretory energy content increased during the feeding trial. No differences were seen in the analysed blood metabolites. Gizzards of the test birds were heavier than gizzards of the control birds. We conclude that the abrupt change from a commercial to a natural diet with the following difference in diet composition affects the partridge's ability to utilise nutrients from food available in the wild. According to our study, a period of six weeks may be inadequate for partridges to get totally adapted to a new diet.