Washington Post Co. profits rose 15 percent in the second quarter compared with the same period last year, excluding nonrecurring gains in 1978 from selling radio station WTOP and other property.

Second-quarter net earnings this year were $15.4 million (98 cents a share), down from $17.2 million ($1.06) a year earlier, the company said yesterday. However, the 1978 earnings included one-time gains of $3.8 million (23 cents a share). Excluding these gains, earnings for the 1978 period would have been $13.4 million.

Revenues of the Post Co. increased about 13 percent to $152.3 million, with all three divisions - newspapers, broadcasting, magazines and books - reporting gains. The Washington firm owns Newsweek magazine, four television stations and three daily newspapers, including The Post.

In the first six months of 1979, Post Co. earnings were $24 million ($1.51 a share) compared with $24.3 million ($1.49 on more shares outstanding) a year ago, including the nonrecurring gains in 1978 mentioned earlier. Revenues increased to $282.6 million from $248 million.

A company statement said both the newspaper and broadcasting divisions had higher pre-tax profits in the recent period but that the magazine and book publishing division reported "a slight decrease."

Martin Cohen, vice president and treasurer, attributed the lower earnings at Newsweek to higher expenses for circulation and distribution as well as spending for new product development.

Earlier this month, Newsweek President Peter Derow confirmed reports that a new monthly magazine is being developed, called "Inside Sports." He said it will be test-marketed next fall in Northeastern states, including the Washington area.

With an initial cover price of $1.50, the magazine will cover professional and amateur spectator sports, "sports as a culture" and individual athletes with an emphasis on the professional team cities of Baltimore, Washington, Philadelphia, Pittsburgh, Boston, Buffalo and New York. John Walsh, formerly with The Post's Style section, has been named editor and Newsweek vice president Dan Capell was appointed publisher.

The Post Co. also said yesterday that since 1975, it has repurchased 4.495 million of its own common shares - including 462, 000 shares in the first half of this year.

Macke Co., a food services and vending firm based in Cheverly, reported yesterday that earnings for its third fiscal quarter ended June 30 rose to $1.6 million (48 cents a share) from $1.27 million (38 cents) a year ago. Sales increased to $66.8 million from $59.4 million.

Nine-month earnings rose to $4.2 million ($1.26) from $3.5 million a year earlier, counting only continuing operations. Income of $50,000 in the year-earlier period from the discontinued Southern Instruments Co. made final net income $3.6 million ($1.08 a share) in the prior nine months. Sales rose to $195 million from $170 million.

Pargas Inc., a Waldorf-based distributor of LP-gas, reported record earnings for the second quarter and first six months of 1979. Pargas earned $1.24 million (33 cents a share) in the recent quarter compared with a loss of $1.14 million a year ago. Sales rose to $35.6 million from $30.5 million.

For the six-month period, Pargas profits were $5.6 million ($1.53 a share) compared with $2.1 million (57 cents) as sales rose to $86 million from $82 million. Coal mining operations contributed to earnings in the recent quarter in contrast with a steep loss a year ago, Chairman William Hill said also that propane profits continue at record levels. CAPTION: Graph, Big Oil Profits, The Washington Post