Martin Marietta Corp. a diversified aerospace, aluminum and construction materials firm based in Bethesda, yesterday reported a slight decline in third-quarter profits. Earnings were up in the period for Dynalectron Corp. and Pargas, Inc., two other large companies with headquarters in the area.

Profits of Martin Marietta fell to $27.4 million ($1.15 a share) from $27.7 million ($1.17) in the same period last year, although sales increased to $363 million from $323 million.

Chairman J. Donald Rauth said results of Martin's aluminum division were not as strong in the third period as earlier in the year because power cutbacks in the Pacific Northwest have forced a reduction in output, and a labour dispute at a rolling mill in Kentucky interrupted operations for a month.

Nevertheless, Martin Marictta's nine-month profits were up substantially to $80.6 million ($3.39) from $60.2 million ($2.55) last year. Sales increased to $1.05 billion from $899 million. Rauth noted that the 1977 pace to date continues to exceed the previous record year of 1974.

Earnings and sales increased at all five divisions during the first nine months, he said. Rauth repeated a forecast made at last spring's annual meeting that 1977 would be the company's most successful year, an accomplishment that now seems to be comfortably within reach."

Dynalectron Corp., a McLean-based electrical engineering, contracting and aviation services firm, reported third-quarter profits of $1.2 million (17 cents a share), up 44 per cent from $330,000 (12 cents) in the 1976 period. Revenues rose to $77.7 million from $51.3 million.

A company statement indicated that each of Dynalectron's three operating groups contributed to third-quarter gains, with growth expected to continue in the current period.

Because of depressed earnings in the first six months, however, nine month profits declined to $1.06 million (14 cents) from $1.95 million (28 cents), as sales increased to $202 million from $144 million.

At the end of the Sept. 30 quarter, Dynalectron had an unbilled backlog of contracts totaling $244 million compared with $232 million a year ago.

Pargas, Inc., a Waldorf-based distributor of liquefied petroleum gas and owner of coal mines, reported third-quarter profits rose to $672,000 (17 cents a share) from $453,000 (10 cents) last year. Revenues increased to $30.7 million from $25.1 million.

Because of the strong quarter usually weak for the propane business, nine-month profits surged to $4.9 million ($1.31) on sales of $106 million compared with earnings of $2.8 million (73 cents) on sales of $85 million.

Pargas also announced yesterday that previously reported negotiations on a possible merger with Energy Resources Corp. of Dallas have been terminated.

NUS Corp. of Rockville reported record third-quarter profits of $451,000 (45 cents a share) compared with a year-earlier loss of $2,000. Revenues rose to $9.3 million from $5.8 million.

A higher volume of business was cited as the primiary factor for earnings growth.

In the first nine months, the engineering and consulting company had profits of $969,000 (97 cents) vs. $21,000 (2 cents) while sales increased to $24 million from $18 million. The NUS backlog of orders on Sept. 30 was $22 million, up from $14 million last year.

Hotel Investors of Kensington reported improved results for both the year and the fourth quarter ended Aug. 3.

Earnings for the year totaled $1.8 million ($1.20 a share compared with $1.6 million ($1.01) in the previous year.Fourth-quarter profits were $426.358 (28 cents) compared with a net loss of $172.914 in the fourth quarter last year.

Both annual and fourth-quarter earnings were after an extraordinary charge of $422.344 (27 cents) resulting fron the refinancing of its unsecured term loan agreement with three banks. Also included in fourth quarter results was a gain of $392.777 (25 cents) on the sale of an investment in a Dallas hotel.

he real estate investment trust, which specializes in hotel investments, also noted thjat negotiations were substantially completed in October for the sale of its investment in a Hilo, Hawaii, hotel. The trust will continue its existing financing for the new borrowers and will purchase of additional facilities. The sale is expected to be completed within a few weeks.