The activities of succinate dehydrogenase (SDR) and lactate dehydrogenase (LDH) as well as the ribonucleic acid levels in the epithelium of both the lactic ducts and alveoli of lactic glands were examined in heifers, close to full term, lactating cows, some of them with subclinical mastitis lesions, and udder biopsies of two cows. The activities of SDH and LDH as well as RNA levels were pronounced in the glandular epithelium of alveoli and in the epithelium of the lesser lactic ducts. All three parameters were also pronounced but at lower levels in the cells of the apical stratum of the two-layer epithelium in the greater lactic ducts. Relationships were found to exist between the enzyme activities and RNA levels and the lactation cycle. Their patterns and manifestations were more or less typical of those areas of the lactic gland which were affected by mastitis.