In its second largest transaction ever, M&Ms maker Mars, which also owns veterinary services businesses such as Banfield Pet Hospital and Pet Partners, is acquiring pet health care services provider VCA (WOOF) in a $9.1 billion deal, including $1.4 billion in outstanding debt, the companies said Monday.

The firms started discussions late last year, according to a source familiar with the deal.

Privately held Mars, based in McLean, Va., is buying VCA for $93 a share, marking a 31% premium over Friday's closing price of $70.77.

"When senior members of Mars' management approached us with an offer to join their organization, it was with mixed emotions that we considered their proposal," VCA CEO Bob Antin, COO Art Antin and SVP of development Neil Tauber wrote in a letter to employees Monday.

Neither VCA's management nor its board sought out the Mars offer, the executives said. "After thorough deliberations, we determined that this is a unique opportunity for all of the company's stakeholders, especially our employees," they said.

The deal is expected to close in the third quarter. Bob Antin will continue to lead VCA post-transaction.

Raymond James analyst Nicholas Jansen said the enterprise value of the deal represents an adjusted 2016 Ebitda multiple of a little more than 18 times.

"This is at a premium to recent private equity multiples in the animal hospital space (12-13x), reflective of the company's robust lab franchise, which we always argued was underappreciated in the market," Jansen wrote in a note on Monday.

Jansen thinks that Mars "found VCA to be an attractive long-term asset, and we would expect the two businesses to complement one another quite nicely."

Shares of VCA traded at $90.62 Monday morning, up 28%.

VCA operates nearly 800 small animal veterinary hospitals in the US and Canada. The company also has a network of veterinary diagnostic laboratories. Its other businesses include medical technology and pet care services.

VCA will continue to be based in Los Angeles and will operate as a business unit within Mars Petcare, alongside Mars' other veterinary services businesses.

Mars' pet care business includes pet nutrition brands such as Royal Canin, Pedigree and Whiskas. Mars also offers pet DNA testing through Wisdom Panel. Last year, Mars purchased pet technology provider Whistle for more than $100 million, TechCrunch reported.

"VCA is a leader across pet health care and the opportunity we see together-for pets, pet owners, veterinarians and other pet care providers -is tremendous," said Mars CEO Grant F. Reid in a news release.

Mars has six business units including pet care, chocolate, Wrigley, food, drinks and symbioscience, which is the Mars' division devoted to nutriceuticals and supplements based on flavanols to help with heart, vascular and cognitive health and the treatment of diabetes and certain forms of cancer.

The VCA deal is Mars' second largest transaction after Mars and Berkshire Hathaway's (BRK.B) - Get Report purchase of gum company Wrigley for about $23 billion in 2008. Mars said in October of last year it bought out a minority stake in its Wrigley subsidiary held by Berkshire Hathaway.

J.P. Morgan & Co. is providing financing for the transaction.

Morgan Stanley & Co. LLC's Ben Frost and BDT & Co.'s San Orr served as financial advisers to Mars. Skadden, Arps, Slate, Meagher & Flom LLP's Howard Ellin, Neil Stronski, June Dipchand, Kyle Hatton, Marissa Smith, Cliff Gross, Paul Schockett, Clifford Aronson, Michael Sheerin, Stephanie Teicher, Neil Leff and Young Park provided legal advice on the acquisition, Simpson Thacher & Bartlett LLP's Jennifer Hobbs and Associate Jennifer Albrecht and Capital Markets Partner Richard Fenyes advised on the debt financing. McDermott Will & Emery's Will Diaz assisted Mars on antitrust matters.

Barclays plc's Todd Richter, Jed Brody and  Evan Matlin  served as financial advisers to VCA, which received legal counsel from Akin Gump Strauss Hauer & Feld LLP's Frank Reddick, Ken Menges, Carlos Bermudez, John Holland, Mike Miller, Ashton Butcher, Kevin Tsai, Paul Hewitt and Corey Roush and Potter Anderson Corroon LLP's Michael Tumas.

A Mars representative was not immediately available for comment and a VCA representative declined to comment beyond the announcement. -

David Marcus contributed to this report