Experiments were carried out with 32 lambs weaned at different age and fed a dry milk replacer as well as with 20 control lambs nursed by their ewes. All lambs were divided into three groups. Those of the first one were weaned at the age of 5 days, those of the second group - at 15 days, and those of the third group - at 20 days. Blood was sampled at the beginning of the test and then for a second and a third time at a 15-day interval in order to study the serum glutamate-oxal-acetate transaminase (SGOT), the serum glutamate pyruvate transaminase (SGPT), the alkaline phosphatase (AP), and the lactate dehydrogenase (LDH). It was established that at the first study in the beginning of the experiment SGOT and SGPT were at a higher level in the test animals than in the control ones. AP was found to rise in both groups with the advancement of age. Lowest values in the activity of LDH were recorded with the two groups at the age of 5 days. No differences were established between the values of LDH in the test and control groups in all investigations. The rise of SGOT and SGPT in the test lambs spoke of adaptation of the enzymes as a result of changes in the metabolic pattern with the lambs of different feeding.