Goats were hemimastectomized during Weeks 5 or 8 of lactation. The milk yield of the remaining gland (the test gland) usually increased in a compensatory fashion after the operation, although the degree of response varied from no response to a 50% increase. The glands of those goats that responded and received no further treatment maintained an increased milk yield for the duration of the lactation. Changes in the size and composition of the gland were investigated by a combination of udder volume and biopsy in some goats. Increases in both gross size and total DNA content (DNAt, a measure of cell number) were observed in all of these, although the timing of the response varied between individuals. Overall, cell population size was maintained for at least 18 weeks after hemimastectomy whereas in control (unoperated) goats DNAt decreased by 35% over a similar period.