1 In this study, we have used Kir6.1/Kir6.2 chimeric proteins and current recordings to investigate the molecular basis of PNU-37883A inhibition of cloned K ATP channels. 2 Rat Kir6.1, Kir6.2 and Kir6.1/Kir6.2 chimeras were co-expressed with either SUR2B or SUR1, following RNA injection into Xenopus oocytes, and fractional inhibition of K ATP currents by 10 mM PNU-37883A reported. 3 Channels containing Kir6.1/SUR2B were more sensitive to inhibition by PNU-37883A than those containing Kir6.2/SUR2B (mean fractional inhibition: 0.70, cf. 0.07). 4 On expression with SUR2B, a chimeric channel with the Kir6.1 pore and the Kir6.2 amino-and carboxy-terminal domains was PNU-37883A insensitive (0.06). A chimera with the Kir6.1 carboxyterminus and Kir6.2 amino-terminus and pore was inhibited (0.48). These results, and those obtained with other chimeras, suggest that the C-terminus is an important determinant of PNU-37883A inhibition of Kir6.1. Similar results were seen when constructs were co-expressed with SUR1. Further chimeric constructs localised PNU-37883A sensitivity to an 81 amino-acid residue section in the Kir6.1 carboxy-terminus. 5 Our data show that structural differences between Kir6.1 and Kir6.2 are important in determining sensitivity to PNU-37883A. This compound may prove useful in probing the structural and functional differences between the two channel subtypes.