The application of cold or colchicine to dividing epidermal cells of the newt larva produces by similar means an arrest of mitosis at metaphase. The cells respond to both agents by complete suppression or by abnormal development of the spindle. The various types of abnormal spindle which are produced can be described in terms of the complete or incomplete inactivation of centromeres, centrosomes or both. Spermatogonia and spermatocytes, unlike the epidermal cells, invariably show complete spindle suppression during cold treatment. The more obvious differences between the effects of the two agents are due (a) to the greater severity of spindle suppression produced by colchicine, and (b) to the slowing up of all cell processes produced by cold. Thus while both treatments lead to the production of relapsed tetraploid cells, these occur more commonly after colchicine treatment. The untreated epidermal cell swells up during anaphase. This swelling is exaggerated in cells held at metaphase under the action of cold or colchicine. The inactivation effects can be described in terms of a tactoid theory, as due to surface changes in centrosomes or centromeres which prevent them orientating or breaking down spindle molecules.