A young Johns Hopkins University graduate smashed through a double-pane window early yesterday and plunged to his death from a millionaire family's posh midtown apartment after night of partying, police and relatives said.

Just before hurtling out the eighth-floor window, Seth Pitman, 23, told friends he "wanted to enter the afterlife" and knew he could "fly," police sources and pals said.

"He was talking to himself, he was praying, he was uncontrollable, I didn't recognize him," his girlfriend Elektra Carras said.

Pitman, a chemical engineer who moved to Brooklyn to pursue a career in music, banged on walls, screamed and tried to kiss a man and a woman during the party at 205 W. 54th St. before his death, pals said.

Falling six stories, he slammed against an air conditioning unit before landing on a second-floor scaffolding, where he was found dead about 1 a.m.

Cops were investigating his death as a possible suicide and awaiting drug and other tests from an autopsy, sources said. But Pitman's mom insisted the Florida native wasn't depressed. She blamed his death on a possible medical condition.

"I know my son trusted Jesus as his Savior. It was something he believed and lived," Donnie Pitman said from her home in Milton, Fla. "I asked his girlfriend, is it possible someone slipped him a drug? She said no, nothing like that happened.

"To hear him running wild, trying to hurt people, breaking through a double-paned glass, it makes no sense."

The tragic party was thrown by the adult children of millionaire Jean Claude Simille, who owns a lucrative beauty and day spa business. Simille said he and his wife, Marie, were in Albany at the time of the bash.

"You know what your children are doing?" he asked reporters. "I feel terrible about this. But I feel good that my children are fine."

Seth Pitman's girlfriend, whom he met at Johns Hopkins nearly four years ago, said she was scared to get close to him while he was acting crazed. At one point, his screams roused a tenant four floors below, the doorman and the superintendent.

"I see this guy flipping out, banging the walls. Then he tried to attack these guys," said the tenant, who gave his name only as Arthur. "He was enraged. There was something that snapped."

Carras said she followed him back inside the Similles' apartment and left him in a bedroom. Moments later, she heard the sound of shattering glass.

"He was alone in the living room for only a few seconds," she said. "People were waiting, hoping he would calm down. He was completely in his own world, but no one thought anything like this was possible."