Suburban Bancorp, based in Bethesda, reported yesterday that its earnings increased 18.5 percent to $26.9 million ($5.38) for the year, up from $22.6 million ($4.74) in 1983.

Suburban also reported an annual growth in assets of 12.6 percent, to $2.9 billion from $2.57 billion. Suburban said its fourth-quarter earnings rose 22.1 percent to $7.7 million ($1.44) from $6.3 million ($1.32) in the same period last year.

Suburban Chairman Robert F. Tardio attributed the strong earnings performance to the growth in loans, which increased 33.8 percent during the year to a year-end total of $1.5 billion.

The company also said that it benefits from its lack of problem loans. Suburban had no nonperforming loans at the end of the year. Additionally, the firm has no international, agricultural or energy loan exposure.

Gray and Co., the largest Washington-based lobbying and public relations firm, said its third-quarter earnings rose slightly to $336,724 (14 cents a share), compared with $332,264 (19 cents) in the same period last year.

Third-quarter revenue for the period ending Nov. 30 was up 92 percent to $5.3 million from $2.7 million in the same period a year earlier.

For the nine months, net income was stable at $840,000, the same amount earned in the first nine months of 1983. Revenue for this period rose about 50 percent to $12.8 million from $8.4 million.

Gray recently acquired a Wellesley, Mass.-based public relations and advertising firm that specializes in high-technology clients. The subsidiary was renamed Gray Strayton International.