Ares Management on Thursday announced its purchase of National Veterinary Associates in Agoura Hills. The Century City private equity firm bought the animal hospital chain from funds affiliated with Summit Partners, which has offices in Boston, London and Menlo Park. National Veterinary’s senior management will retain a minority stake. Terms of the purchase were not disclosed. The transaction is expected to close in the third quarter of the year. National Veterinary has more than 240 animal hospitals operating in California and 38 other states. The company was founded in 1996. The announcement was made after markets closed. Ares Management’s share price fell 6 cents, less than a percentage point, to close at $19.02 on the New York Stock Exchange.