1. DDT1 MF-2 smooth muscle cells responded to the bath application of histamine or ATP with an increase in the cytosolic Ca2+ concentration ([Ca2+]c) and the whole-cell K+ current, IK(BA). 2. In cell-attached patches, histamine (100 microM) activated currents through a 200 pS K+ channel (‘BKA’ channel). In the absence of agonists, the BKA channel was activated by excision of the patch. Both histamine and patch excision increased the channel activity (NPo; where N is the number of channels per patch and Po is the open probability) by reducing the long closures between the bursts of openings. 3. In inside-out patches, the BKA channel had a conductance of 201 +/- 4 pS (symmetrical solutions of 150 mM KCl, 2 mM MgCl2 and 2 mM EGTA). Replacement of K+ in the patch electrode by Na+, Li+ or Cs+ prevented the flow of inward currents and reduced the outward K+ conductance to 113 pS. 4. NPo was insensitive to changes in [Ca2+]c from 10 nM to 1 microM. NPo was also not modified either by cytosolic Na+, ATP, GTP, GTP gamma S, dithiothreitol or TEA (10 mM) or by extracellular 4-aminopyridine (5 mM), glibenclamide (20 microM) or TEA (10 mM). The BKA channel was blocked by 5 mM intracellular BaCl2 or by 10 nM extracellular iberiotoxin. 5. In cell-attached patches, BKA channel activity could be induced by 1 microM cytochalasin B, applied either through the patch pipette or in the bath solution. The effects of cytochalasin B, of patch excision, or of histamine on NPo were not additive but saturative. 6. Whole DDT1 MF-2 cells had resting potentials of -10 mV, dominated by the chloride conductance; the resting potential changed to -82 mV when the K+ conductance was increased by cytochalasin B or by histamine. The effects of cytochalasin B and histamine on IK(BA) were not additive but saturative. 7. We discuss the hypothesis that the interaction between the cytoskeleton and the BKA channel promotes the long channel closures; depolymerization of F-actin may constitute a mechanism by which the agonists histamine or ATP disinhibit BKA channel activity.