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1. We have measured the ATP dependence of KATP channel activity, and the effect of various metabolites on this relationship, in inside‐out membrane patches isolated from rat ventricular myocytes. 2. The inhibition of KATP channel activity by ATP could be described as a sigmoid function of [ATP] with a Hill coefficient (HATP) of 2 and a half‐maximal inhibition at an ATP concentration (Ki, ATP) of 25 microM, in the presence of 0 mM, or 0.5 mM, total [Mg2+]. The non‐hydrolysable ATP analogue, AMP‐PNP, also inhibited the channel with Ki, AMP‐PNP = 60 microM and HAMP‐PNP = 2. 3. Acidosis caused a small, but significant, increase in Ki, ATP from 25 microM at pH 7.25 to 50 microM at pH 6.25, but phosphate and lactate were without effect (at 20 mM) on channel activity. 4. In the absence of ATP or Mg2+, ADP3‐ inhibited channel activity with Ki, ADP = 275 microM, and HADP = 1.2. Other purine and pyrimidine …