MCI Communications Corp. yesterday reported its first quarterly loss since 1986, $168 million in red ink that the company attributed to a one-time charge it took in speeding up the modernization of its long-distance telephone network.

MCI's stock dropped $1.12 1/2 to $29.87 1/2 on the news. But many analysts thought investors were focusing not on the loss, which is not expected to recur, but on lower-than-expected growth in revenue from quarter to quarter at the Washington-based company.

Gene Gabbard, MCI's chief financial officer, said that in late September the rate of revenue expansion had slowed at the same time as an overall slowdown in the long-distance industry. But he said that in general he saw the growth rate as "good news."

Through the 1980s, MCI was one of Wall Street's great growth machines, expanding helter-skelter in a newly deregulated long-distance market that now has more than $55 billion in sales annually. But investors have wondered how long it could keep up its expansion, particularly with the economy slowing, and have scrutinized quarter-to-quarter figures closely.

Yesterday, MCI announced that revenue for the quarter ended Sept. 30 totaled $2 billion. "The revenue number was lighter than anyone expected," said William Deatherage, first vice president at Dean Witter Reynolds Inc. in New York.

Calculating the growth rate was complicated by the fact that during the quarter, MCI acquired Telecom USA, a regional long-distance firm based in Atlanta. Deatherage, however, put the expansion from second-quarter revenue at about 2 percent, saying analysts had expected 4 percent.

MCI's revenue was up 20 percent in the quarter from year-ago levels of $1.7 billion, and rose 19 percent in the first nine months, to $5.7 billion from $4.8 billion.

Early in the quarter, MCI announced that it would take a one-time charge against profits to retire aging microwave transmission equipment by 1991 rather than 1993 as previously planned. It moved more of its calls onto fiber optic lines brought to the company by Telecom USA and to lines leased from third parties.

The charge totaled $550 million. Without it, after-tax profits would have been $180 million (71 cents a share) in the quarter, according to Gabbard. That compared with $165 million (62 cents a share) a year ago. Costs of the $1.25 billion acquisition of Telecom USA also helped depress earnings, Gabbard said. "Our earnings would be higher by a few cents {a share} if it weren't for Telecom," he said.

He noted that operating income at the company, the country's second-largest long-distance carrier, grew 16.7 percent over the previous quarter and 16.1 percent over the third quarter of 1989.

John S. Bain, telecommunications analyst at the securities firm of Raymond James & Associates Inc. of St. Petersburg, Fla., said MCI remained in good shape. "The company did something in the double-digit range of growth {annually}, and these days that's good," he said.

For the first nine months of 1990, MCI's revenue grew 19 percent, to $5.68 billion from $4.76 billion. Earnings dropped almost 62 percent, to $172 million (59 cents) from $450 million ($1.69).