Power supplies used in field-cycling nuclear resonance experiments are described. These are used to switch an air coil from high field to low field as rapidly as possible, then back to high field after a short time. This sequence must be repeated every few seconds or minutes. An example of a simple regulator is described, and methods of output switching with relays are discussed. A circuit is described which delivers up to 10 kG (15 A) in a 2.6 cm i.d. superconducting coil with a switching speed of 5×105 G/sec and an accuracy probably as great as 1 G. Speed is achieved by automatic connection, during up or down switching, of energy storage capacitors across the magnet, operating at much higher voltage than the steady-state voltage level across the magnet and pass transistors. © 1968 The American Institute of Physics.