Dr. Harry Whelan, a pediatric Neurologist at the Medical Hospital of Wisconsin in Milwaukee and professor of Neurology at the Medical College of Wisconsin, explains the operation of light-emitting diodes (LEDs) developed for use in space plant growth experiments and now adapted for use in photodynamic therapy, a technique in which light from the LEDs activates light-sensitive, tumor-treating drugs. The technique has been used in at least two surgeries on brain tumors. The LED project was one of several NASA exhibits at AirVenture 2000 sponsored by the Experimental Aircraft Association in Oshkosh, WI.