1. To study the mechanism of regulation of sulphonylurea sensitivity in ATP-sensitive K(+) (K(ATP)) channels, we used the inside-out patch clamp technique in guinea-pig ventricular myocytes. 2. In the absence of nucleotides, the half maximal concentration of tolbutamide inhibition of K(ATP) channels (IC(50)) was 0.4 mM, and it decreased to 0.1 mM when 0.1 mM ATP was added. 3. Increasing the ADP concentration from 0 to 0.1 and 0.3 mM in the absence of ATP shifted the IC(50) from 0.4 to 5.3 and 11.4 mM, respectively. Increasing the ADP concentration further to 1 and 3 mM conversely reduced the IC(50) to 9.5 and 4.4 mM, respectively. 4. In the absence of Mg(2+) and ADP, the IC(50) was calculated to 16.6 mM which was found to be less, 12.3, 5.1 and 2.5 mM, respectively, when the ADP concentration was increased to 0.1, 0.3 and 1 mM. 5. The IC(50)s for tolbutamide obtained at various concentrations of ADP in the presence of Mg(2+) were best fitted by equations reflecting a model that assumed two binding sites for ADP; one is a high affinity site that reduces the sensitivity to the sulphonylurea, while the other is a low affinity site that increases such sensitivity. Dissociation constants calculated for ADP to sites 1 and 2 were 2.6 microM and 46.7 mM, respectively. In the absence of Mg(2+), data were fitted by equations corresponding to a single site model (site 2); the dissociation constant for ADP was 25.0 mM. 6. It is concluded that ADP modifies tolbutamide sensitivity by binding to two sites. The high affinity site is strongly Mg(2+)-dependent, whereas the low affinity site is Mg(2+)-independent.