1. We have studied the electrophysiological properties of cloned ATP-sensitive K+ channels (KATP channels) heterologously expressed in Xenopus oocytes. This channel comprises a sulphonylurea receptor subunit (SUR) and an inwardly rectifying K+ channel subunit (Kir). 2. Oocytes injected with SUR1 and either Kir6.2 or Kir6.1 exhibited large inwardly rectifying K+ currents when cytosolic ATP levels were lowered by the metabolic inhibitors azide or FCCP. No currents were observed in response to azide in oocytes injected with Kir6.2, Kir6.1 or SUR1 alone, indicating that both the sulphonylurea receptor (SUR1) and an inward rectifier (Kir6.1 or Kir6.2) are needed for functional channel activity. 3. The pharmacological properties of Kir6.2-SUR1 currents resembled those of native beta-cell ATP-sensitive K+ channel currents (KATP currents): the currents were > 90% blocked by tolbutamide (500 microM), meglitinide (10 microM) or glibenclamide (100 nM), and activated 1.8-fold by diazoxide (340 microM), 1.4-fold by pinacidil (1 mM) and unaffected by cromakalim (0.5 mM). 4. Macroscopic Kir6.2-SUR1 currents in inside-out patches were inhibited by ATP with a Ki of 28 microM. Kir6.1-SUR1 currents ran down within seconds of patch excision preventing analysis of ATP sensitivity. 5. No sensitivity to tolbutamide or metabolic inhibition was observed when SUR1 was coexpressed with either Kir1.1a or Kir2.1, suggesting that these proteins do not couple in Xenopus ocytes. 6. Our data demonstrate that the Xenopus oocyte constitutes a good expression system for cloned KATP channels and that expression may be assayed by azide-induced metabolic inhibition.