Synchronized Tetrahymena pyriformis were immersed for varying periods in systematically varied concentrations of Colcemid. With increasingly higher concentrations of Colcemid the cells displayed progressively longer division delays. Cells suspended in inorganic medium were more sensitive to Colcemid than cells in nutrient medium. Possibly Colcemid forms a complex with some constituent(s) of the nutrient medium, thus rendering the agent less effective. A transition from maximum sensitivity to insensitivity was evident when Tetrahymena were exposed to Colcemid at 45–55 min after EH. Moreover, the population became separated into 2 groups during this transition period: an ‘early-maturing group’ dividing with minimal delays; and a ‘late-maturing group’ dividing with maximal delays. On Colcemid reversal, cells divided with no apparent loss in synchrony 75–85 min after Colcemid removal. Biochemical studies revealed that DNA, RNA, and protein synthesis were inhibited approx. 90, 70, and 40%, respectively, in cells continuously exposed to 4 × 10 −4 M Colcemid. Furthermore, the release of cells from a Colcemid block effectively released the block to DNA, RNA, and, to some extent, protein synthesis. Colchicine also inhibited DNA synthesis to a greater degree than RNA or protein synthesis. In addition to affecting the integrity of microtubules, the action of Colcemid and colchicine may be mediated through the inhibition of macromolecular synthesis associated with the cell cycle.