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An active satellite-ground coordinated space plasma experiment was conducted from May to December, 1982, in which electrons were precipitated from the radiation belts into the ionosphere by the controlled injection of VLF signals from ground-based transmitters. The results confirm the hypothesis that electrons can be precipitated from the radiation belts by ground-based VLF transmitters, and they provide information relating to the effects of such precipitation on the ionosphere. The ionization produced in the atmosphere of the northern hemisphere at L = 2.3 by the modulated signals from ground-based VLF transmitters was shown to be as great as one ion pair/cm3 s at 80 km altitude. The ionization at comparable positions produced by naturally occurring electron precipitation varies greatly, and can be as low as 0.1 ion pair/cm3 s, but is also sometimes larger than 100 ion pairs/cm3 s at times of lightning flashes.