Washington Gas Light Co. earnings rose for the past 12 months, while a seasonal loss in the second quarter was reduced from year-earlier levels. Separately, Dynalectron Corp. yesterday reported record earnings for the second quarter of 1981.

WGL said second-quarter losses were reduced to $3.7 million from a loss of $4.5 million in the 1980 period, as revenues increased to $79.2 million from $67.3 million. The local natural gas distrubtion firm usually suffered a loss in the April-June quarter because of low gas consumption in the nonheating months.

For the full year ended June 30, WGL earnings rose to $18.3 million ($3.49 a share) from $15.2 million ($2.85), and revenues rose to $509 million from $419 million. A gas company statement attributed the higher profits to increased rates and a slightly colder winter. WGL and subsidiaries in the D.C. region had 552,825 meters in service at mid-year, an increase of about 9,500 from a year ago.

Dynalectron Corp. of Mclean said second-quarter profits were a record $4.36 million (46 cents a share) compared with $801,000 (11 cents) a year earlier, as revenues rose to $122.7 million from $106 million. The 1981 results included a one-time gain from resolution of insurance claims related to heavy-construction activities, allowing a recovery of costs previously written off as expenses.

For the first half of the year, the diversified energy, technical services and electrical contracting company earned a record $5.4 million (63 cents), up 198 percent from $1.8 million (25 cents) a year earlier. Sales increased to $230.5 million from $203.2 million.

Maryland Cup Corp. posted record sales and profits in the quarter and nine months ended June 30. The Owings Mills, Md., manufacturer of paper and plastic food-service products said third-quarter earnings were $8.9 million ($1.33 a share) versus $7.5 million ($1.12), a gain of 19 percent. Sales rose 11 percent, to $178 million. For the nine months, Maryland Cup earned $17 million ($2.54) on sales of $453 million, compared with earnings of $13.3 million ($1.98) and sales of $404 million a year ago.