1. Milk secretion in one mammary gland of goats was temporarily inhibited by intramammary treatment with colchicine at weeks 6, 12, 18, 24 and 30 of lactation. The magnitude of the inhibition was similar at all stages when compared with milk yield before the experiment. 2. The untreated gland partially compensated for the decreased rate of milk secretion by significantly increasing its own rate of secretion. There was no significant effect of stage of lactation on this response. 3. The rapidity of the compensatory response suggests that existing secretory cells increased their rate of secretion, and therefore that the mammary secretory cells are not secreting at their intrinsic maximum at any stage of lactation. 4. In later stages of lactation (weeks 18, 24 and 30) the increases in the untreated gland were more pronounced in afternoon milkings (8 h milking interval) than in morning milkings (16 h interval. At week 30 only, the rate of secretion before treatment was similarly lower in the 16 h period. It is proposed that an additional limitation is imposed on secretion during the relatively long period of milk accumulation as lactation advances and declines. 5. The results are discussed in relation to rate-limitations on milk secretion with respect to substrate supply and possible systemically active mammary factors.