MCI Communications Corp. plans to buy 24 satellite transponders from Hughes Communications Inc., a transaction industry sources say is worth about $200 million.

Representatives of the two companies, while refusing to reveal the precise price of the package, said the purchase represents the largest single transponder deal in history. Transponders are the units on satellites that receive signals and beam them to earth stations.

MCI spokesman Gary Tobin said that between the Hughes purchase and MCI's plans to construct a series of fiber-optics networks, the company will have increased its communications transmission capacity by 50 percent during the next four years. Tobin said the company's total satellite program, which includes currently leased satellite space from RCA Corp., will cost between $200 million and $300 million.

Hughes, the communications arm of privately held Hughes Aircraft Co., has sold transponders on its Galaxy I satellite to cable networks.

That satellite is to be launched in June. MCI has purchased half the 24 transponders on each of two similar Hughes satellites scheduled for launch in September 1983 and in July 1984, provided the Federal Communications Commission approves the plan. MCI will begin using the Hughes satellites within 60 days of launch.

MCI will use the satellites for voice and data communications among Hughes' earth stations in seven cities. The capacity is designed to prepare MCI "for the anticipated increase in demand for our services which will occur" next year, MCI Chairman William McGowan said.

At that time, American Telephone & Telegraph Co. is scheduled to begin offering MCI access to additional telephones, such as those with rotary dials, rather than push button controls, under terms of the Bell System's consent agreement with the government.