Abstract: An improved method of trimming resistors by metal migration by local heating (18,21) of the resistor (23) to allow a reduced level of electric current to be used. The resistor is then trimmed by application of short pulses of high current causing metal migration (22). Each pulse decreases the resistance progressively until a desired final value is achieved. The reduced level of current of the improved method requires a reduced voltage across the resistor thus extending the earlier method to allow higher valued resistors, or lower breakdown voltages. The localized heating (18,21) is also used to anneal the migrated metal after trimming to increase the long term stability of the resistor (23). The intensity of the localized heating (18,21) can be varied to induce a desired shape in the migrated metal (22).