NEW YORK—North Castle Partners of Greenwich, CT, and J.W. Childs Associates of Boston, have signed a definitive agreement to sell Equinox Holdings Inc. for $505 million to The Related Companies, a New York City-based real-estate development firm.

Equinox operates upscale fitness clubs in New York, Chicago, Los Angeles, San Francisco and South Florida. The sale is expected to close in January 2006.

Harvey Spevak will continue to lead Equinox as CEO and president, and the current Equinox management team will remain intact, The Related Companies said.

In December 2000, North Castle Partners and J.W. Childs Associates partnered with Harvey Spevak and company management to acquire Equinox Holdings Inc. At that time, the company was a New York centric business. Since 2000, Equinox has opened 21 new clubs in four new markets and more than doubled its membership. Revenues have grown from $63 million in 2000 to $168 million for the 12-month period ended September 2005.

Related and Equinox already have a luxury condominium project in the works together on the Upper West Side of Manhattan, where Equinox will operate a new 35,000 square foot club.