Colchicine, the plant alkaloid, produced a dramatic decrease in milk flow when infused into the udder of the goat. The compound (1 to 5 mg) dissolved in 5 ml of water was inserted into one side of the under via the teat canal. Such treatments consistently caused a depression in milk yield from the infused side with maximum at 36 h and substantial reversal by 72 to 96 h. Milks from both the infused and uninfused sides of the udder were essentially normal in composition (fat, protein, and lactose). However, globulins and riboflavin were elevated in milks from the infused side. The plant alkaloid, vincristine, produced effects on milk secretion similar to those of colchicine but at dosages roughly one-tenth the latter. The two substances had no effect on the amount of milk from the uninfused side of the udder. Experiments employing [carbon-14] colchicine revealed that less than 20% of the infused colchicine is secreted in the milk. Both the secretion of fat globules and the emptying of secretory vesicles by the lactating cell are inhibited by colchicine indicating that a portion of the cell population is turned off from secretion. Plant substances such as colchicine and vincristine may at times limit yields of milk, especially in grazing ruminants.