Viewers of new Channel 4 series Space Cadets, which fools contestants into thinking they are in space, have fallen to two million.
Space Cadets contestants were chosen for their suggestibility
Its launch show drew an average of 2.6 million viewers but figures fell to an average of two million on Tuesday.
It was beaten by programmes on BBC One and Two, ITV1 and Five.
Channel 4 was "not disappointed" by the show's performance, a spokeswoman said. "It was always going to be a brave show to do," she added.
"The audience has stayed loyal. They've stayed pretty consistent," she said.
"It's an innovative programme that has never been done before. In terms of ratings we're doing fine... it's not a Big Brother-type show."
Tuesday was Space Cadet's worst weekday performance - figures had already dipped to 1.2 million at the weekend.
The 10-day series, presented by Johnny Vaughan, will end on Friday when contestants will discover that the "shuttle" they are in is actually a simulator in a TV studio.
The concluding episode of ITV1 thriller Secret Smile topped the ratings from 2100 GMT on Tuesday, attracting 6.2 million viewers.
The last series of Channel 4 reality series Big Brother drew an average of 6.7 million viewers.