Analysis of DNA content of intact cells by laser induced cytofluorometry permits dynamic studies of the cell cycle in the synchronously dividing eukaryote Euglena gracilis. In this manner, the effects of zinc deficiency and cadmium toxicity on in situ DNA synthesis and cell division of this organism have been studied. In the G-1 hase of the cell cycle, prior to initiation of DNA synthesis, the DNA content of synchronously growing E. gracilis is characteristic of cells with an ureplicated genome. In S phase there is a progressive increase in DNA content which leads to genome duplication as the cells enter G2. In the subsequent mitosis all cells divide. Cytofluorometric definition of DNA content serves as a standard of reference to study variables which alter or block each stage of the cell cycle. Growth in zinc dificient media inhibits cell division. The DNA content of such zinc dificnet cells is characteristic of a population of cells blocked in S/G2 with a small fraction in G1. Moreover, cells synchronized in G1 and placed in zinc deficient media do not progress into S phase. Cadmium also inhibits cell division, and the DNA content of these blocked cells is three to four times greater than that of cells in G1. Zinc is essential for the biochemical events of the premitotic state which include initiation of DNA synthesis, DNA synthesis, and progression from G-2 to mitosis. Cadminum-induced derangements of the cell cycle include alterations in regulation cellular DNA content.