Kay Jewelers said it negotiated an $80 million line of credit from a group of banks and that three principal shareholders on its board of directors invested an additional $5 million in the beleaguered Alexandria-based company. The retail jewelry chain, with 494 stores nationwide, had lost one-third of its bank lines of credit in May, but said the new credit arrangement will satisfy its needs for the rest of the year. Kay's stock closed yesterday at $10.37 1/2 in trading on the New York Stock Exchange, up 67 1/2 cents.

AT&T said it will reduce its evening interstate long- distance rates by about 5 percent and daytime rates by about a half-percent on July 1 as part of a $300 million overall price cut. It also will reduce the prices of operator-assisted calls, U.S.-Canada calls and its Reach Out America optional long-distance calling plan.

Mobil concluded agreements with the Nigerian National Petroleum Corp. and the Nigerian government for a $400 million development plan for the Oso offshore oil field, which is estimated to contain nearly 500 million barrels of reserves.

Gibraltar Savings, which was the nation's 10th-largest thrift before it failed and was taken over by the U.S., is being purchased by Security Pacific Corp. Terms of the deal are expected to be disclosed after a definitive agreement is signed.

Fluor Corp. has been awarded a contract to oversee the near-doubling of Saudi Arabia's oil production capacity. Officials at the Irvine, Calif.-based engineering and construction company and Saudi Arabia's state-owned Aramco put no exact value on the contract, but published reports quoted analysts as saying it could be worth $2 billion to $5 billion.