American Management Systems of Arlington late last week reported a sharp drop in net income in the three months ended Sept. 30 and a loss for the first nine months of the year.

The computer systems and services and management consulting firm showed a loss of $731,000 before taxes in the latest quarter. However, after income tax credits against new investments made this year the company was able to show a profit of $9,000 (1 cent a share) in the three months. This was down sharply from net income of $634,000 (42 cents) in the same period last year.

In the nine months to Sept. 30 the pretax losses totalled $804,000, compared to income before taxes of $3.2 million in the same period of 1980. The after-tax loss for the latest three quarters was $81,000, compared to net income of $1.79 million ($1.20 a share) in the comparable period of 1980.

However, revenues were up in the latest quarter and the nine months by 7 percent and 10 percent, respectively, to $16.7 million and $48.1 million, the company said.

American Management Systems has been hit this year by higher interest costs and corporate expenses, including opening a new office in Houston, Tex., and consolidating its main facilities into a single new building as leases were running out on the previous buildings, the company said.

Consolidating the main facilities cost the company about $1.2 million in the first nine months of this year, most of which fell in the second and third quarters, the company said. It will cost another $400,000 in the fourth quarter.

American Management's stock price dropped slightly last week to $13.25. However, in the last four weeks the stock price has risen 26.2 percent, with growing volume in the last three weeks, reaching 22,000 last week. Senior officals in the company could not be reached for comment over the weekend on the trading.

In other results:

Overnite Transportation Co. said third-quarter operating profits rose to $6.6 million ($1 a share) from $5.1 million (78 cents) a year ago as revenues increased to $77.4 million from $60.4 million. A one-time gain for the Richmond trucking company in the recent quarter made final net income $11.9 million ($1.80), while a one-time loss last year produced a net loss of $5.2 million.

McLean Savings and Loan reported net income for the second quarter of $10,500 (12 cents) compared with $30,500 (34 cents) a year ago. For the first six months of the fiscal year the savings and loan institution had earnings of $36,000 (39 cents) compared with $23,000 (25 cents) a year before. In the same period, savings deposits rose to $18.308 million, 21 percent higher than a year before.