1. The effects of various inhibitors of the mitochondrial electron transport chain on the activity of ATP-sensitive K+ channels were examined in the Cambridge rat insulinoma G1 (CRI-G1) cell line using a combination of whole cell and single channel recording techniques. 2. Whole cell current clamp recordings, with 5 mM ATP in the pipette, demonstrate that the mitochondrial uncoupler sodium azide (3 mM) rapidly hyperpolarizes CRI-G1 cells with a concomitant increase in K+ conductance. This is due to activation of KATP channels as the sulphonylurea tolbutamide (100 µM) completely reversed the actions of azide. Other inhibitors of the mitochondrial electron transport chain, rotenone (10 µM) or oligomycin (2 µM) did not hyperpolarize CRI-G1 cells or increase K+ conductance. 3. In cell-attached recordings, bath application of 3 mM sodium azide (in the absence of glucose) resulted in a rapid increase in KATP channel activity, an action readily reversible by tolbutamide (100 µM). Application of sodium azide (3 mM), in the presence of Mg-ATP, to the intracellular surface of excised inside-out patches also increased KATP channel activity, in a reversible manner. 4. In contrast, rotenone (10 µM) or oligomycin (2 µM) did not increase KATP channel activity in either cell-attached, in the absence of glucose, or inside-out membrane patch recordings. 5. Addition of sodium azide (3 mM) to the intracellular surface of inside-out membrane patches in the presence of Mg-free ATP or the non-hydrolysable analogue 5'-adenylylimidodiphosphate (AMP-PNP) inhibited, rather than increased, KATP channel activity. 6. In conclusion, sodium azide, but not rotenone or oligomycin, directly activates KATP channels in CRI-G1 insulin secreting cells. This action of azide is similar to that reported previously for diazoxide.