The degree of inhibition of semiconservative DNA replication induced by nickel chloride (NiC12) was analyzed by radiolabeled-thymidine incorporation alone or with cesium chloride (CsCl) density gradient centrifugation. The onset and duration of this NiZf-induced inhibition was time-and concentrationdependent, but the degree of inhibition was not. A maximal reduction in the rate of DNA synthesis was observed within the first hour of treatment with 2.5 mM NiC12, which was the highest noncytotoxic concentration utilized. After six hours, 500 pM and 1 mM as well as 2.5 mM NiClz all produced the same 50% to 60% reduction in 13H]-thymidine incorporation into DNA. The inhibitory effect of nickel ions on DNA synthesis was reversible. The rate of DNA synthesis following a 500 pM or 1 mM NiClZ treatment began to increase after washout of nickel, but a six-hour exposure of cells to 2.5 mM NiC12 produced a sustained 50% to 60% suppression of DNA synthetic activity for at least 36 hours. At all concentrations of NiC12 used in this study, some inhibition of DNA synthesis persisted for at least 48 hours, but by 72 hours after treatment, the rate of 13H1-thymidine incorporation was actually 10% above the control. Examination of autoradiographic slides of cells treated with 2.5 mM NiClZ for six hours demonstrated a 60% reduction of silver grains, but there was no preferential reduction in the quantity of grains in the nucleolus or any other region. Cesium chloride density gradient analysis of the replication of nucleolar DNA in cells treated with 2.5 mM nickel supported the autoradiographic findings. The inhibitory effect of NiC12 on DNA replication was prevented by the addition of magnesium chloride (MgC12) to cells maintained in a simple saltslglucose medium (SGM). This effect did not appear to be due to an antagonism of the cellular u take of nickel by MgZf, since the maximally effective dose of Mg2+ reduced 69Niz+ Manuscript