Effects of La~93+) and Gd~(3+) on Ca~(2+) influx were investigated in rat hepatoma H-35 cells by measuring the initial rate of~(45)Ca~(2+) uptake. It was found that the maximum initial rate of Ca~(2+) uptake was increased six- to ten-fold at low concentrations of La~(3+) and Gd~(3+). Kinetic analyses by measuring the initial rate of Ca~(2+) influx at different external Ca~(2+) concentrations indicated the existence of two intracellular exchangeable components in the basal Ca~(2+) system, with low and high affinities for Ca~(2+),and only one class of Ca~(2+) binding sites was observed in the La~(3+)- or Gd~(3+)-treated cells. For high affinity, La~(3+) and Gd~(3+) increased both kinetic parameters K_m and V_(max of basal Ca~(2+) influx. La~(3+) and Gd~(3+) compete directly with Ca~(2+) for Ca~(2+) binding site for low affinity. The kinetics is competitive. Effects of La~93+) and Gd~(3+) on Ca~(2+) influx were investigated in rat hepatoma H-35 cells by measuring the initial rate of~(45)Ca~(2+) uptake. It Was found that the maximum initial rate of Ca~(2+) uptake was increased six- to ten-fold at low concentrations of La~(3+) and Gd~(3+). Kinetic analyses by measuring the initial rate of Ca ((2+) influx at different external Ca 2+ areas indicated that the existence of two intracellular exchangeable components in the basal Ca 2+ 2+ system, with low and high affinities for Ca 2+, and only One class of Ca2+ binding sites was observed in the La3+- or Gd3+-treated cells. For high affinity, La3+ and Gd3+ Increased both kinetic parameters K_m and V_(max of basal Ca~(2+) influx. La~(3+) and Gd~(3+) compete directly with Ca~(2+) for Ca~(2+) binding site For low affinity. The kinetics is competitive.