Tacrolimus is widely used for immunosuppressant therapy including various organ transplantations. One of its main side effects is hyperglycemia due to reduced insulin secretion, but the mechanism remains unknown. We have investigated the metabolic effects of tacrolimus on insulin secretion at a concentration that does not influence insulin content. Twenty-four hours exposure to 3 nM tacrolimus reduced high glucose (16.7 mM)-induced insulin secretion (2.14 ± 0.08, control vs. 1.75 ± 0.02 ng/islet/30min, 3 nM tacrolimus, P<0.01) without affecting insulin content. In dynamic experiments, insulin secretion and NAD(P)H fluorescence during a 20 min period after 10 min high-glucose exposure were reduced in tacrolimus-treated islets. ATP content and glucose utilization of tacrolimus-treated islets in the presence of 16.7 mM glucose were less than in control (ATP content: 9.69 ± 0.99, control vs. 6.52 ± 0.40 pmol/islet, tacrolimus, P<0.01; glucose utilization: 103.8 ± 6.9, control vs. 74.4 ± 5.1 pmol/islet/90 min, tacrolimus, P<0.01). However, insulin release from tacrolimus-treated islets was similar to that from control islets in the presence of 16.7 mM -ketoisocaproate (KIC), a mitochondrial fuel. Glucokinase activity, which determines glycolytic velocity, was reduced by tacrolimus treatment (65.3 ± 3.4 control vs. 49.9 ± 2.8 pmol/islet/60 min, tacrolimus, P<0.01), while hexokinase activity was not affected. These results indicate that glucose-stimulated insulin release is decreased by chronic exposure to tacrolimus due to reduced ATP production and glycolysis derived from reduced glucokinase activity.