THE under-five set has a message for former “Blue’s Clues” host Steve – don’t let the (cartoon) door hit you on the way out!

Last Monday’s 90-minute “Blue’s Clues” episode introducing Steve’s replacement, Joe, snagged 1.9 million viewers between the ages of 2 to 5 – the biggest preschool audience in basic cable history.

Psychologists and parenting experts had worried that youngsters might not adjust well to Steve leaving the show, and Nickelodeon went out of its way to devote 90 minutes to Steve’s departure.

But, based on the num bers, all the hand-wringing wasn’t necessary.

In Monday’s episode, called “Meet Joe,” it was explained that Steve was heading off to college – and that his brother, Joe (played by actor Donovan Patton) would take his place.

Steve appeared in the episode to help ease the transition.

The “Meet Joe” episode aired from 7 to 8:30 p.m. and was the highest-rated entertainment program among the preschool set this year on either broadcast or basic cable.

The show was also the highest-rated Nick Jr. prime-time special ever, out-rating “Blue’s Big Pajama Party,” which aired in October ’99, by 16 percent.