1. The effects of papaverine (10(-5)-2 X 10(-4) M) were studied on the evoked electrical and mechanical activity of the guinea-pig ureter smooth muscle. In normal conditions the action potential consists of an initial spike followed by further spikes superimposed on a plateau phase. Papaverine reversibly decreased the duration of the plateau of the action potential, blocked the associated spikes, greatly reduced the amplitude of the contraction but enhanced the initial component of the action potential. 2. Papaverine did not change the membrane potential and had little effect on the membrane resistance. 3. Tetraethylammonium (5 mM), which blocks the delayed outward K current, did not prevent the decrease in the duration of the plateau nor the decrease of the contractile response caused by papaverine. 4. In Na-free solution the duration of the action potential was decreased until only a single spike was seen, due to suppression of the plateau. An effect of papaverine could not be observed under these conditions. 5. Mn2+ ions (1 mM) completely suppressed the spike component and tension while the plateau component was substantially increased. Papaverine in the presence of Mn2+ reversibly blocked the generation of the action potential. When Mn2+ ions were added to Na-free solution the duration as well as the amplitude of the spike was increased. Again, papaverine reversibly blocked the generation of the action potential. 6. Noradrenaline (10(-4) M) and histamine (10(-5) M) in normal conditions prolonged the duration of the action potential plateau and increased both the duration and amplitude of the concentration. Papaverine again blocked the plateau and greatly reduced the contractile response. 7. Papaverine caused the relaxation of KCl-induced contractures, preferentially blocking the tonic component. 8. It is suggested that the inhibitory action of papaverine on ureter smooth muscle results from its specific blockade of the 'slow' Na/Ca channels responsible for the generation of the plateau component of the action potential.