The effect of arachidonic acid (AA) on the delayed rectifier K+ current (IK) was evaluated in rat pulmonary myocytes by using the whole-cell patch-clamp technique. Externally applied AA (50 mumol/L) caused a membrane depolarization, averaging 16 mV in six cells. AA (1 to 50 mumol/L) caused a dual effect on IK. First, AA accelerated the rate of IK activation, increasing current amplitude at the beginning of voltage step. Second, AA caused a marked acceleration of current decay, thereby reducing IK amplitude measured toward the end of the depolarizing steps. These effects were not prevented by indomethacin or nordihydroguaiaretic acid, blockers of cyclooxygenase and lipoxygenase, respectively. AA did not affect the voltage dependence of current activation or inactivation. The magnitude of the inhibitory effect on IK was correlated with the number of double bonds but was independent of tail length in fatty acids containing between 14 and 22 carbons. Linoleic acid (18:2, cis-9,12) inhibited IK much more than did its trans-stereo-isomer, linolelaidic acid. Arachidonyl alcohol, which is uncharged, and arachidonyl coenzyme A, which does not 'flip' across the cell membrane, were less effective than AA in inhibiting IK; this effect of fatty acids may therefore require passage across the cell membrane. The enhancement of early IK was mimicked by the protein kinase C (PKC) stimulator 1-oleoyl-2-acetyl-sn-glycerol (10 mumol/L), was suppressed by ATP removal from the pipette solution, and was blocked by PKC inhibitors chelerythrine (10 mumol/L) and staurosporine (100 nmol/L). This effect may therefore require PKC-dependent phosphorylation.