In the washroom.Presumably somewhere near. Why? Need a potty break?
Mr Scott's Guide to the Enterprise by Shane Johnson, page 43, shows a toilet is next to the bridge level airlock where Spock comes aboard during Star Trek: The Motion Picture. The airlock & toilet are accessible from the port side turbo lift which has a separate door to this area.
The Enterprise-D's restroom was supposedly on the starboard side forward of the door to the Conference Lounge. And presumably there is 1 in the bridge ready room. TrekFan1984 Ends (talk) 03:50, February 6, 2016 (UTC)