A solid‐state constant current supply which operates in the current range of 1 to 100 &mgr;a with load resistances up to several hundred megohms is described. The system is battery operated, and allows one side of the load resistance to be grounded. The current is controlled by a gallium arsenide photoresistor in series with the load. The load current is balanced against a standard current and the difference is fed to a chopper‐amplifier which drives a small lamp focused on the photoresistor. The gain of the amplifier is sufficiently high that a very small unbalance in current will drive the lamp to full output; thus, the photoresistor keeps the load current very near the standard current for any value of load resistance from zero up to the dark resistance of the photoresistor, typically about 8×108ohms.