The inside-out configuration of the patch-clamp method was used to study the effects of trypsin on the activity of ATP-sensitive potassium (K-ATP) channels from isolated mouse pancreatic beta-cells. Trypsin (20 micrograms/ml) irreversibly enhanced channel activity around twofold by reducing the interburst intervals without altering the burst kinetics. No effect on the single channel conductance or the inward rectification produced by internal Mg2+ was observed: however, the protease did reduce the inhibitory effect of Mg2+ on channel activity. Trypsin both prevented rundown of K-ATP channel activity and reactivated the channels after complete rundown. These effects of trypsin were absent in the presence of trypsin inhibitor. The protease also reduced the inhibitory effect of ATP on channel activity, increasing the dissociation constant from 7 to 49 microM. Trypsin removed the activating effect of ADP (0.1 mmol/l) on channel activity and reduced the inhibitory effect of tolbutamide (0.5 mmol/l). Carboxypeptidase A did not activate K-ATP channels in excised patches, although it was able to slightly reactivate channels after complete rundown, whereas chymotrypsin increased K-ATP channel activity but it did not produce reactivation. The effects of papain were similar to those of trypsin.