Responses to adrenaline in mouse peritoneal macrophages were investigated with perforated and cellattached patch-clamp recording, and with a combination of the perforated-patch recording and fura-2 fluorescence measurements. Extracellularly applied adrenaline induced a transient outward current (4-10 s in duration, 100-500 pA in amplitude) at -40 mV associated with a marked increase in conductance. The adrenaline-induced current [Io (Adr)] reversed polarity near -80 mV. The reversal potential depended distinctly on the external K + concentration but not on external C1-concentration. Removal of external Ca 2+ did not affect Io(Adr) within 2-4 min but subsequent responses to adrenaline were progressively depressed. In contrast, treatment with an intracellular Ca z+ chelator, the acetoxymethyl ester of 1,2-bis-(2-aminophenoxy)ethane-N,N,N',N'-tetraacetic acid completely abolished Io(Adr). Furthermore, Io(Adr) was blocked by bath-applied quinidine and charybdotoxin, but not by tetraethylammonium or apamin. Extracellular application of an al-adrenoceptor agonist phenylephrine and of noradrenaline mimicked Io(Adr). On the other hand, Io(Adr) was antagonized by a non-selective a-adrenoceptor antagonist phentolamine (0.2 IxM) and an al-adrenoceptor antagonist prazosin (0.2 ~M), but was not affected by an a2-adrenoceptor antagonist yohimbine (1 ~tM) or a fl-adrenoceptor antagonist propranolol (1 tiM). Cell-attached single-channel recordings with the pipette solution containing 145 mM KC1 revealed the activation of single-channel currents with a conductance of 40 pS during application of adrenaline outside the patch. Parallel measurements of membrane current and fura-2 fluorescence in the same cell demonstrated a correlation between the rise in [Ca 2+ ]i and an increase in K + conductance. Therefore, it is concluded that adrenaline activates a Ca2+-dependent K + conductance by release of Ca 2+ from internal stores through an activation of an a~-adrenoceptor.