BDM International Inc. of McLean and Kay Corp. of Alexandria reported sharply higher profits yesterday, but Preston Trucking Co. posted lower earnings because of extraordinary write-offs required by federal trucking deregulation laws.

The Preston, Md., freight hauler deducted $747,000 from its quarterly earnings to cover the value of "operating rights" that give it exclusive authority to serve particular routes. The rights will lose their value because new federal law takes away the exclusive franchises and opens the business to competition.

Before the write-off, Preston Trucking reported net earnings for the quarter ended Sept. 30 of just under $2 million (63 cents a share) on revenues of $43.6 million. After the extraordinary deduction, the line earned $1.2 million (39 cents) compared with earnings of $2.2 million (70 cents) on revenues of $41.9 million in the same period a year ago.

For the first nine months of the year, Preston earned $3.8 million ($1.21) on revenues of $128.6 million, down from $4.3 million ($1.37) on revenues of $118.4 million last year.

BDM International, parent of a group of consulting and technical services companies, reported third-quarter earnings of $690,000 (39 cents a share), up 32 percent from the $522,000 (31 cents) of last year. Revenues increased to $21.6 million from $16.1 million.

For the nine-month period, BDM earnings increased 29 percent to $1.9 million ($1.10) from $1.5 million (87 cents) and sales climbed 28 percent to $59.1 million from $46.2 million.

Kay Corp., an international trade and retail jewerly firm with headquarters in Alexandria and its main operations in New York, reported operating income for the quarter of $1.3 million (40 cents a share), up from $413,000 (10 cents) in the same period a year ago.

The company also had a nonrecurring gain of just over $4 million ($1.31) from the sale of the assets of Stockton Elevators, a subsidiary of Pvo International. Kay acquired PVO, then disposed of the Stockton operation. Including the gain from the sale, Kay posted profits of $5.2 million ($1.31) for the quarter.

For this nine-month period, Kay showed a loss before the Stockton sale of $540,000 on revenues of $701 million compared with earnings of $2.1 million (60 cents) on sales of $525 million a year ago. Including the extraordinary item, Kay earned $4.3 million ($1.34) for the nine months.

Ensco Inc. of Springfield reported reduced earnings for the year ended June 30 of $536,537 ($1.14 a share) compared with $610,594 ($1.32) a year earlier. Revenues were $15.7 million compared with $13.6 million. Ensco is engaged in research, development and engineering.