OBJECTIVE: In the production of miRNAs, pre- miRNAs are cleaved by the enzyme Dicer1 to derive functional miRNAs. We have utilized a mouse model with b-cell specific disruption of the Dicer 1 gene (RIP-CreDicer^-/-) to investigate the effect of miRNAs on insulin secretion. METHODS: Insulin secretion from intact islets was determined by ELISA. Membrane currents were recorded using the standard whole-cell configuration of the patch-clamp technique. Changes in cell capacitance was used to measure exocytosis. Ultrastructural analyses of the b-cells were performed by electron microscopy. RESULTS: Glucose-stimulated insulin secretion (20mM) was normal in 2 week old mice but reduced by >70% in 5 and 8 week old RIP-CreDicer^-/- compared to control (p<0.001). Trains of 500 ms membrane depolarisations from –70 mV to 0 mV was applied on b-cells from 8-9 week old mice to investigate exocytosis. The response to the first two pulses correlating to the first phase of insulin secretion, was significantly lower in the RIP-CreDicer^-/- mice (19±14 fF vs. 51±17 fF; p<0.05) compared to control. The reduction was not due to reduced voltage-dependent Ca²⁺ currents (peak current 38±3 pA vs. 47±5 pA; p=n.s). Electron microscopy on 11 week old mice showed a dramatic loss of b-cells in RIP-CreDicer^-/- . Analysis of the remaining cells revealed a 50% reduction of total granulas and a 60% reduction of docked granulas. CONCLUSION. Loss of Dicer 1 and the miRNAs it produces affects b-cell exocytosis insulin secretion and b-cell survival and can thereby play a role in the development of diabetes.