Experiments were carried out to study the induction of endoreduplication by colchicine in Chinese hamster cells cultivated in vitro. The cells that endoreduplicate are those that, at the moment of treatment, are in late-S and, in particular, in G2. The endoreduplication cycle consists of two periods of synthesis (S1 and S2), as already noted by Schwarzacher & Schnedl [42], separated by an intervening period that we call G? The S2 synthesis begins in a highly synchronous manner, without the cells having gone into a c-mitosis. The quantity of endoreduplicated cells induced is proportional to the 3.5th root of the colchicine concentration, above a threshold value, and does not depend on the duration of the treatment. When the cultures are treated twice with colchicine, the second treatment is also able to induce endoreduplication and, after it, there appear double endoreduplicated cells (with quadruplochromosomes).