Porter Novelli CEO Gary Stockman is leaving the firm after four years at the helm of the Omnicom unit.

Stockman, who is staying on through late July, took the reins in January 2008 from Helen Ostrowski after he joined the firm with its acquisition of Copithorne and Bellows in 2000.

"For more than a decade I've been committed to the transformation Porter Novelli leads for our clients every day. After nearly five years as CEO, it's time for a new experience," Stockman said in a statement.

He follows the exit of chief marketing officer Michael Goldberg, who left this month after a brief stint at PN to return to the advertising sector.

Julie Winskie, president of PN who oversaw a retrenchment of the agency’s operations in late 2008, stepped down in April. 

Ostrowski is now a senior advisor at APCO Worldwide.

PN has not yet named a permanent replacement for Stockman. Senior partner Michael Ramah, a 25-year PN veteran, will work with the departing CEO during a transition and assume the duties of acting CEO on Stockman's departure, PN said.

Anthony Viceroy is president and CFO and PN has eight senior partners.

Parent Omnicom said it supports PN and the search for a new chief. Dale Adams, CEO of OMC's Diversified Agency Services, which houses PN, said: "The Porter Novelli history and its future make it a valued member of the Omnicom family and we will fully support the leadership transition and appointment of a new chief executive officer."