Peoples Drug Stores Inc., trying to become a larger force in the nation's drugstore industry, yesterday announced it has reached an agreement to buy Rea & Derick Inc., a 134-store Pennsylvania chain.

The acquisition -- at an undisclosed price -- will enlarge Peoples' presence in the mid-Altantic and eastern Midwest, where the company has a total of 621 stores. It will also make Peoples, a recently acquired subsidiary of the Canadian tobacco and food conglomerate Imasco Ltd., a billion-dollar company in terms of revenue.

"This acquisition represents an extension of our commitment to the North American retail drugstore industry," noted Imasco's chairman and chief executive officer Paul Pare, announcing the agreement.

"It is an excellent fit for Peoples," said Peoples' Chairman Sheldon W. Fantle. "This will give us a dominant market position . . . in the state of Pennsylvania," he said. While Peoples has stores in Harrisburg, Lancaster, York, Hershey, New Castle and other Pennsylvania cities, Rea & Derick has significantly more, he noted.

Still more acquisitions may follow, Fantle indicated. "We're always looking for acquisitions now that we are part of a company that wants to grow . . . Imasco is willing to let us go ahead and move as rapidly as we can."

Imasco acquired Peoples for $400 million last spring.

Fantle said he doubted that Peoples would buy any more chains until the company can digest the Rea & Derick acquisition. But he added, "If something good came up, nothing would stop us."

Fantle noted that the acquisition will enable Peoples to meet more rapidly its goal of being a 1,000-store chain and a $2 billion company by 1990.

When the acquisition is completed -- which is expected to be next week -- Peoples will become a 755-store chain, with revenue near $1.15 billion. Peoples' annual sales total $950 million; Rea & Derick's, $200 million.

Fantle said he has been interested in buying Rea & Derick for more than four years. But until recently its parent company, American Stores Co. of Salt Lake City, had refused to sell the Pennsylvania chain.

However, American Stores changed its mind as a result of its recent $1 billion purchase of Jewel Co.'s Inc. With more than $1 billion in debt, the company wanted to redeploy its assets. As a result, it decided to spin off some of its properties, Fantle said.

American Stores turned over the chain to Shearson/Lehman American Express Inc., which solicited bids. Within four hours after Peoples submitted its final offer yesterday -- along with six other bidders -- it was notified that it had won the chain.

Other companies reported to be bidding included Sherwin Williams Co., the parent of Gray Drug Fair. Sherwin Williams officials could not be reached to confirm this report.