The converting enzyme catalyzes the conversion of angiotensin I to angiotensin II. Ang II is the key component of the renin-angiotensin-aldosterone (RAA) system, regulating water-electrolyte balance in newborn calves. Captopril is an inhibitor of the angiotensin-converting enzyme. The aim of this study was to examine the effect of captopril-induced reduction of convertase activity on copper and iron concentrations of blood plasma in calves. The experiment was carried out on 10 Holstein-Friesian female calves, during the first week of life. Copper and iron concentrations in blood plasma were examined before and after captopril administration (0.5, 1, 2, 4, and 6 hours after giving the inhibitor) on subsequent days of the experimental period. The results demonstrated that the copper concentration of blood plasma increased with age. On the seventh day, the copper concentration stabilised at the level observed in adult cattle. Measured before captopril administration, the iron concentration in blood plasma changed: the highest iron concentration was observed on the first day of life, which was followed by a decrease on the third day, and thereafter an increase on the seventh day. These changes may significantly influence the neonatal adaptation of newborn calves, particularly hemopoiesis efficiency. Captopril did not cause statistically significant changes in plasma copper concentration in calves. However, the reduction of angiotensin convertase activity induced by captopril administration resulted in a drop of plasma iron concentration, observed already within 1-2 hours after administration of the inhibitor, and especially within two days post partum. The results indicate that an efficient mechanism maintaining a constant concentration of selected minerals may involve changes in the reabsorption of these minerals from the system fluids to tissues.