1. The perforated patch and conventional whole-cell recording techniques were used to study the action of flufenamic, mefenamic and niflumic acid on calcium-activated chloride and potassium currents in rabbit portal vein smooth muscle cells. 2. In K-conditions at a holding potential of -77 mV flufenamic acid and mefenamic acid decreased the amplitude of spontaneous transient inward currents (STICs, calcium-activated chloride currents, ICl(Ca)) in a concentration-dependent manner. The potency sequence was niflumic > flufenamic > mefenamic acid. 3. At -77 mV 1 x 10(-5) M flufenamic acid increased the STIC exponential decay time constant (tau). At higher concentrations the STIC decay was described by 2 exponentials with an initial decay (tau f) faster than the control tau value and a second exponential (tau s) which had a time constant slower than the control tau value. Low concentrations of mefenamic acid had no effect or decreased the tau value whereas in higher concentrations biphasic currents were recorded. 4. In K-free conditions the inhibitory effect of both flufenamic and mefenamic acid on STIC amplitude was greater at +50 mV compared to -50 mV, showing that the effect of these agents was voltage-dependent. 5. In cells held at 0 mV in K-containing conditions the fenamates reduced both the frequency and amplitude of spontaneous transient outward currents (STOCs, calcium-activated potassium currents, IK(Ca)). The concentration range to produce these effects was higher than that to decrease STIC amplitude and the potency sequence was flufenamic > niflumic > or = mefenamic acid. 6. All these compounds in concentrations greater than 5 x 10(-5) M evoked a 'noisy' potassium current at 0 mV which reached a maximum after approximately 3 min. This current was readily reversible on washout of the drug and could be elicited several times in the same cell. The current-voltage relationship of the fenamate-evoked current exhibited pronounced outward rectification characteristic of IK(Ca). 7. The current evoked by 2 x 10(-4) M flufenamic acid and 5 x 10(-4) M niflumic acid was not affected by 1 x 10(-5) M glibenclamide but was markedly inhibited by 1 x 10(-3) M tetraethylammonium. Furthermore, large currents were activated by flufenamic and niflumic acid in the presence of caffeine and cyclopiazonic acid (an inhibitor of the sarcoplasmic reticulum Ca-ATPase) to deplete intracellular Ca-stores. 8. Conventional whole-cell recording was performed with pipette solutions in which the ability to buffer changes in intracellular calcium was varied by altering the concentration of the calcium chelator (2-aminophenoxy)-ethane-N,N,N',N'-tetraacetic acid (BAPTA). Flufenamic acid (2 x 10(-4) M) and niflumic acid (5 x 10(-4) M) both evoked large outward currents when recordings were made with either 1 x 10(-4) M or 1 x 10(-2) M BAPTA. Furthermore, bathing the cells in nominally calcium-free extracellular solution did not reduce the amplitude of the evoked currents. 9. It is concluded that both flufenamic and mefenamic acid inhibit ICl(Ca) by a mechanism similar to niflumic acid, possibly open channel blockade. Furthermore, at concentrations greater than 5 x 10(-5) M all three fenamates inhibited STOC activity and evoked directly an outward current which resembled IK(Ca).