The effect of low concentrations (1-10 microM) of nickel (Ni2+) was studied on the spontaneous contractile activity of isolated rat portal vein preparations. To distinguish between presynaptic and postsynaptic actions, Ni2+-induced changes of isometric force development evoked either by electrical field stimulation (80V/5 cm; 1 msec; 1-32 Hz) or by exogenous norepinephrine (NE) were compared. Ni2+ inhibited spontaneous isometric force development, decreased basal tone but significantly increased the frequency of contractions. The experimental findings that inhibition of the effect of selective stimulation of adrenergic nerves was significantly more pronounced than the depression of contractions evoked by exogenous norepinephrine indicate that in addition to postsynaptic actions, Ni2+ influences presynaptic mechanisms (i.e. NE-release) in this vessel preparation.