Lyle Schwartz, one of Madison Avenue’s most influential executives, has left the agency where he helped guide strategy for many years – the latest departure of a senior executive from WPP’s large GroupM media-buying operation over the course of the past several months.

In a brief interview, Schwartz said he was leaving after 37 years of working for GroupM and some of its predecessors.  “After a period of time, I’ll reassess opportunities in the industry, or not, depending on what people may be looking for and what I want to do,” he said.

“Lyle Schwartz will be leaving the company,” GroupM said in a statement. “We wish him the very best in his future endeavors and thank him for his many contributions to our company, our culture, our people, and our clients.” Representatives of GroupM did not respond immediately to follow-up queries.

Advertising Age previously reported his departure.

Schwartz was named president of investment at GroupM in 2016, replacing another veteran, Rino Scanzoni. The two played an instrumental role in 2007 in pushing the TV industry to adopt so-called “commercial ratings” that would help count people who were making more use of DVRs to skip past ads and give advertisers a  more accurate sense of audience for various programs. More recently, Schwartz tried to press media companies to adopt a single measure to count viewers of their content, no matter what screen they used. It’s a goal that has eluded the media industry so far, despite may different companies’ efforts.

He was given a new post, chief integration officer of GroupM’s U.S. operations, last year, charged with refining operations. But a previous executive, Tim Castree, had devised that postition, and has since departed the company.

GroupM, which guides billions of dollars in ad spending for companies like Hasbro and Colgate-Palmolive, is one of the biggest media buying agencies in the world. and comprises units such as Mindshare, Essence and Wavemaker. But it has been plagued by the turnover of senior executives, including Kelly Clark and Tim Castree. The revolving door at the big media firm spins as its parent company, WPP of the U.K., has been working to reverse a drop in stock and re-orient itself under Mark Read, the CEO who took over the company following a controversial parting of the ways with its founder. Sir Martin Sorrell.

Schwartz will not be with GroupM as it begins to hold critical discussions with TV networks as part of the industry’s annual “upfront” market., when TV networks try to sell the bulk of their advertising inventory in advance of the coming programming season.

He expects this year’s ad-sales discussions to be intense. “The amount of audience that is migrating to alternate devices, and the industry’s inability to measure it and quantify value?” he asked. “It may not be a bad time to sit on the sidelines and watch.”