Because in vitro secretion is calcium dependent, a study was carried out to ascertain the effect of local anesthetics (LA) on glucose-induced insulin secretion by pancreas pieces. Different classes of LA affected insulin secretion differently. Nonantiacetylcholine type LA with uncharged amino groups (holocaine, propranolol) depressed glucose-induced insulin secretion progressively as their concentrations were raised (10−3 to 1 mM). The inhibition was more marked at lower compared to higher calcium concentrations. The antiacetylcholine type LA (mepivicaine, lidocaine) with charged tertiary amino groups had complex effects on insulin secretion. At low concentrations (10−4 mM) they acted as anticholinergic agents in their inhibitions of Mecholyl-stimulated insulin secretion. Complete blockade of Mecholyl-stimulated secretion was found at calcium concentrations of 0.5 and 3.0 mM. This concentration of LA did not, however, depress glucose-induced insulin secretion. At an LA concentration of 10−3 mM inhibition of glucosestimulated insulin secretion occurred, which was not reversed by higher concentrations of calcium. At still higher LA concentrations (0.05 to 1.0 mM) the inhibitory effect on glucose-induced insulin secretion diminished, disappeared and became a stimulatory effect, which was abolished by the addition of atropine.