MCI Communications Corp.'s second-quarter earnings fell 50 percent from a year earlier, while the Bell regional companies reported mixed results yesterday.

MCI, the nation's second-largest telephone company, earned $8 million (3 cents per share), compared with $16 million (6 cents) in the second quarter of 1986. Revenue rose 1 percent to $952 million from $943 million a year earlier.

In spite of the decline, "We are extremely pleased with the results for a quarter which we expected would be the most difficult of the year," MCI's president, Bert Roberts, said in a prepared statement.

Among the Bell regional phone companies that reported their results yesterday, Philadelphia-based Bell Atlantic Corp.'s profit rose 6.1 percent, while profits fell 6.4 percent at Atlanta-based BellSouth Corp. and fell 3.8 percent at San Francisco-based Pacific Telesis Group.

For the first six months of the year, MCI earned $33 million (12 cents a share), down from $36 million (14 cents) a year earlier. Revenue rose 9 percent to $1.91 billion from $1.76 billion.

Net income for the latest quarter included a pretax gain of $6 million from previously negotiated antitrust settlements and a tax credit of $1 million, counted as an extraordinary item. For the six-month period, earnings included $18 million in pretax gains from antitrust settlements and a tax credit of $6 million.

MCI reduced its rates by 9 percent March 1 to remain cheaper than industry leader American Telephone & Telegraph Co., which dropped its rates in response to a federally required reduction in access charges that long-distance companies pay to local phone companies.

MCI said the reduced access charges did not offset its March price cut, but a more recent round of reductions in the charges are expected to offset MCI's 5 percent rate cut that will take effect Aug. 1.

Telephone analyst John Bain of Shearson Lehman Bros. Inc. said the second-quarter earnings were better than many industry observers had expected.

"Everything was slightly better than we expected," he said. "We had been looking for a loss for the year of 9 cents per share. Now, we're projecting 17 cents earnings per share for the year."

The quarter's earnings were helped by improved collections, benefits from systems designed to curtail fraudulent use of MCI's network and reduced expenditures for leased facilities, the company said.

Bell Atlantic said its results were on target as profit rose to $326.1 million ($1.64 a share), from $307.4 million ($1.54) a year earlier. Revenue rose 6.8 percent to $2.64 billion from $2.47 billion.

For the six-month period, Bell Atlantic said its profit rose 6.1 percent to $633.6 million ($3.18), from $597.3 million ($2.99) a year earlier. Revenue rose 5.6 percent to $5.11 billion from $4.83 billion.

Pacific Telesis, hurt by a rate cut at Pacific Bell, said its profit fell to $272.3 million (63 cents) from $283 million (66 cents). Revenue edged up 1 percent to $2.28 billion from $2.26 billion a year earlier.

Earnings per share were restated to reflect a 2-for-1 stock split on March 25, 1987.

For the six-month period, Pacific Telesis reported a 3.4 percent decline in profit to $538.2 million ($1.25), from $556.9 million ($1.30) a year earlier. Revenue fell marginally to $4.4765 billion from $4.4779 billion.

BellSouth said an early-retirement program and faster depreciation hurt its profit, which fell to $382.3 million (80 cents), from $408.5 million (87 cents) a year earlier. Revenue rose 3.4 percent to $3.03 billion from $2.93 billion.

Per-share amounts reflect a 3-for-2 stock split on Feb. 5.

For its first half, BellSouth said profit rose less than 1 percent to $834.7 million from $828.5 million a year earlier. Earnings per share fell to $1.74 from $1.78 because of an increase in outstanding shares. Revenue rose 4.2 percent to $5.98 billion from $5.74 billion.