The effect of hydroxylamine sulfate (HAS) on the nuclear cycle and the induction of endoreduplication in Zea mays root tips have been studied. The duration of the nuclear cycle was lengthened approx. 50%. An increase in G2 and a rapid reduction of the mitotic index after a 2 h HAS treatment led to the proposal that HAS blocked the transition of nuclei from late S to G2. HAS-induced polyploidy followed from the initial induction of endoreduplication during interphase. A hypothesis for induction of endoreduplication based on a feed-back control mechanism is proposed. Experimental evidence to support the hypothesis has been presented.