Apollo 10 astronauts John Young, command module pilot, foreground, and Eugene A. Cernan, lunar module pilot, background, suit up this morning in preparation for the dry portion of the Countdown Demonstration Test [CDDT] which ended successfully with simulated ignition of their Saturn V rocket at 1:01 p.m. today. Young, Cernan and Stafford, entered the spacecraft about three hours before simulated ignition in a dress rehearsal for the May 18 launch of Apollo 10.
Source: http://images.ksc.nasa.gov