Recent studies indicate that zinc activates p70 S6 kinase (p70^S6k) by a mechanism involving phosphatidylinositol 3-kinase (PI 3-kinase) and Akt (protein kinase B). Here it is shown that phenanthroline, a zinc and heavy metal chelator, inhibited both amino acid- and insulin-stimulated phosphorylation of p70^S6k. Both amino acid and insulin activations of p70^S6k involve a rapamycin-sensitive step that involves the mammalian target of rapamycin (mTOR, also known as FRAP and RAFT). However, in contrast to insulin, amino acids activate p70^S6k by an unknown PI 3-kinase- and Akt-independent mechanism. Thus the effects of chelator on amino acid activation of p70^S6k were surprising. For this reason, we tested the hypothesis that zinc directly regulates mTOR activity, independently of PI 3-kinase activation. In support of this, basal and amino acid stimulation of p70^S6k phosphorylation was increased by zinc addition to the incubation media. Furthermore, the protein kinase activities of mTOR immunoprecipitated from rat brain lysates were stimulated two- to fivefold by 10-300 µM Zn²⁺ in the presence of an excess of either Mn²⁺ or Mg²⁺, whereas incubation with 1,10-phenanthroline had no effect. These findings indicate that Zn²⁺ regulates, but is not absolutely required for, mTOR protein kinase activity. Zinc also stimulated a recombinant human form of mTOR. The stimulatory effects of Zn²⁺ were maximal at ~100 µM but decreased and became inhibitory at higher physiologically irrelevant concentrations. Micromolar concentrations of other divalent cations, Ca²⁺, Fe²⁺, and Mn²⁺, had no effect on the protein kinase activity of mTOR in the presence of excess Mg²⁺. Our results and the results of others suggest that zinc acts at multiple steps in amino acid- and insulin cell-signaling pathways, including mTOR, and that the additive effects of Zn²⁺ on these steps may thereby promote insulin and nutritional signaling.