MCI Communications Corp. more than tripled its profits in the first quarter, but across town Federal National Mortgage Association reported second-quarter losses that doubled those of the year before.

MCI, the D.C. company that competes with the Bell System's long-distance telephone service, reported yesterday that net income for its first quarter ended June 30 was $36.5 million (73 cents a share) on revenues of $185.1 million compared with net income of $10.7 million (18 cents) on revenues of $86 million for the first quarter last year.

MCI's earnings report was on target with the optimistic forecasts of Wall Street analysts who follow the telecommunications firm. The only thing preventing the company from even faster growth is the firm's inability to expand capacity more rapidly, analysts said yesterday.

"In the first quarter, we saw the highest growth yet in our residential telephone service, and you can tell from our ads that this is turning into a big part of our market," said MCI Chairman William G. McGowan.

"With this kind of momentum, we are expanding capacity as fast as we can, and we'll spend $500 million on capital equipment this year versus $260 million last year," he said.

MCI reported net income for the year ended March 31 of $86.5 million ($1.81), and Goldman Sachs and Co. analyst Daniel F. Zinsser said he expects the company to earn $3.75 a share this year and $6 a share by 1984. Zinsser said the MCI strategy of underpricing the Bell System by 30 to 50 percent in the long-distance market will enable the firm to continue gaining market share.

Meanwhile, FNMA reported second-quarter losses of $43.1 million compared with a net loss of $18.5 million last year.

For the first half of 1982, Fannie Mae's net loss totaled $86 million compared with a net loss of $40.1 million in 1981.

Higher borrowing costs adversely affected performance in the second quarter, according to FNMA Chairman David O. Maxwell. These higher costs were responsible for a $15 million increase in portfolio loss to $169.7 million for the quarter, Maxwell said.

Maxwell cited a threefold increase in Fannie Mae's mortgage purchases and commitments for the first half of this year as evidence of the corporation's support of the housing market, and of its determination to add substantial volumes of high-yielding assets to its portfolio of mortgage loans.

The second-quarter losses resulted despite an increase of $10.6 million over the previous quarter in the fees Fannie Mae charged individual lenders, Maxwell said. These fees resulting from transactions with lenders rose in the second quarter to a record $80.3 million, bringing fee income for the first half of the year to $150 million versus $22.9 million in the first half of last year.