A series of in vitro trials have been conducted to determine what relationship exists between zinc and the insulin mechanism. There were no significant differences of the zinc contents in total pancreases excised from the rats thirty minutes after the intraperitoneal glucose dose or from the non-dosed controls. When pancreases were cut into three pieces, and treated in three ways: Incubated without glucose (IWG) followed by isolation of the islets; incubated with glucose (IWG) followed by isolation of the islets; islets were isolated, then incubated with glucose (IWG). The zinc content in the islets was significantly higher (p < 0.01) in the first group than the others. The insulin release and zinc movement were studied in zinc-deficient status. In both Experiment 3 and 5 each pancreas was divided into two, one was treated IWG, one was treated IWOG. There were significant (p < 0.01) differences of zinc content in the islets between lWG and IWOG groups in zinc-supplemented pair-fed and ad lib. controls but not in zinc-deficient rats. In Experiment 5 the zinc contents in the islets of IWOG group were significantly (p < 0.01) lower in zinc-deficient rats than in zinc-supplemented pair-fed rats. Insulin release from the islets incubated with glucose was significantly (p < 0.01) less in zinc-deficient group than in zinc-supplemented pair-fed group. Glutathione contents of the pancreases were measured in Experiment 3, no significant difference was found between zinc-deficient group and both zinc-supplemented groups. Rats were intraperitoneally dosed with glucose or with saline in Experiment 6. After 30 minutes pancreases were excised and the isolated islets were treated with IWG or IWOG. There was no significant difference of zinc content in the islets treated IWOG in glucose-dosed and saline0dosed groups, but significant (p < 0.01) difference was found in the islets treated IWG. Insulin release from the islets treated IWG was significantly (p < 0.01) less in glucose-dosed group than in saline-dosed group. It is concluded that zinc is released from the islets as a component of insulin on glucose stimulation, zinc-deficiency results in an impairment of insulin release or synthesis, and insulin secretion was reduced in the isolated islets taken from animals prestimulated with glucose in vivo.