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An experiment lasting 42 d was performed in 4 consecutive stages on 6 healthy Friesian cows during mid-lactation. Mean values for the different components of mammary secretion during normal lactation were established. Milking was then suspended on all quarters from d 1-14. The mean values for lactate increased 20-to 30-fold over the mean value for normal lactation. Over the same period the leucocyte count (polymorphonuclear leucocytes and lymphocytes) also increased, whereas the mean values for glucose and the estimated redox potential decreased. From d 15-28 milking was resumed on one half of the udder and from d 29-42 milking was also resumed on the other udder-half. When milking was resumed the above-mentioned changes were reversed, taking 5-7 d to reach values obtained during normal lactation in the udder-half which had not been milked for 14 d, but at least 14 d in the udderhalf which had not been milked for 28 d. The changes in the levels of lactate, glucose, leucocyte count and estimated redox potential are discussed in relation to changes in the secretory activity of the mammary gland (lactose, /Mactoglobulin and epithelial cell count), permeability changes of the mammary epithelium (Na + , K + and serum albumin) and the immune defence mechanism in the udder (immunoglobulins). The results indicate that lactate is formed during anaerobic glycolysis by the leucocytes in the mammary secretion, most probably due to reduced blood flow to the udder and the accumulation of secretion in the gland and teat cisterns on cessation of milking.Most studies on function and control of the bovine mammary gland have dealt with the gross composition of milk or with changes in the levels of tissue metabolites and enzymes during the onset of lactation, normal lactation and involution when changes in mammary secretory activity are greatest (Peaker, 1975;Fleet et al. 1975; Schanbacher & Smith, 1975). However, in terms of the physiology of the bovine mammary gland, the diagnosis of sub-clinical mastitis and the production of high quality milk, it is important to elucidate the phenomenon of premature regression. Premature regression is the premature degeneration of the lactating udder epithelium