A simple and rapid method was developed for the K(+) ions determination employing a flow injection system using a flow-through electrode based on the naturally-occurring antibiotic ionophore nonactin occluded in a polymeric membrane. The nonactin ionophore was trapped in poly(ethylene-co-vinyl acetate) (EVA) matrix (40% w/w in vinyl acetate) and dispersed on the surface of a graphite-epoxy tubular electrode. The plasticizer-free all-solid-state potassium-selective electrode showed a linear response for K(+) concentrations between 5.0 x 10(-5) and 5.0x10(-2) M (r=0.9995) with a near-Nernstian slope of 51.5 mV per decade, when Tris-HCl buffer (pH 7.0;0,1 M) was employed as a carrier. The potentiometric-FIA system allows an analytical frequency of 120 samples per hour with a precision of 3.6%. The relative standard deviations (R.S.D.) for K(+) determination in pharmaceuticals samples, without any previous treatment, were lower than 4.0%, comparable to those obtained by flame photometry. Ammonium is the main analytical interference and the electrode response time was 5 s at 25 degrees C. The useful lifetime of the tubular sensor is longer than 3 months in continuous use.