The uptake of nickel ions by the cyanobacterium Anabaena cylindrica was studied. Nickel transport was dependent on the membrane potential of the cells and the rate of uptake was decreased in the dark or by the addition of inhibitors, including uncouplers and electron transport inhibitors, which decreased or abolished the membrane potential of cells. The transport process obeyed hyperbolic kinetics, with a high affinity (apparent Km = 17 +/- 11 (SEM) nM) and low turnover number (maximum velocity = 22.3 +/- 5.4 (SEM) pmol h-1 mg dry wt-1 of cells or flux rate of 3.1 nmol h-1 m-2 of plasma membrane surface area). The process was also apparently specific for Ni2+, the rate being unaffected by the presence of a range of other metal ions in large excess. Equilibrium experiments showed that, over a range of nickel ion concentrations, the cells concentrated Ni2+ by a factor of 2700 +/- 240 (SEM)-fold, corresponding to a chemical diffusion potential for Ni2+ of 101 mV. It was concluded that the cells transport nickel ions by a carrier-facilitated transport process with the concentration factor for the ions being determined by the cell membrane potential according to the Nernst equation.