To obtain small size, high speed ultraviolet sources, we studied the ultraviolet (UV) generation process and efficiency of gallium nitride (GaN) blue light emitting diodes (LEDs). The blue and UV emissions follow a four-level recombination model. Depending on the pump pulse amplitude, the UV-to-blue generation ratio increases and then saturates with increasing pump pulse duration. High efficiency, up to 450 μW UV power at 380 nm, can be obtained from a 1.2 mW blue LED.