A tough new drunk-driving law, expected to produce about 1,000 additional arrests each year, goes into effect in Washington at midnight tonight.

The new law doubles penalties for persons who refuse to take breath or blood tests and sharply increases fines for repeat drunk-driving offenders. Other changes, such as scaling back penalties for first offenders and allowing juries to be told when a defendant refuses to take an alcohol test, make it easier to prosecute drunk drivers.

The new statute comes at a time of increased concern locally and nationwide about drinking and driving. In July, both Virginia and Maryland stiffened penalties for drunk driving, and Maryland raised its drinking age from 18 to 21.

Some of the changes in the D.C. law may appear subtle and procedural, but, Deputy Corporation Counsel Geoffrey M. Alprin said yesterday, "what this law says is you better be very careful driving on the streets of Washington."

Under the new law, persons arrested with blood alcohol levels at 0.1 percent or higher can be convicted of driving while intoxicated even if no other evidence concerning the defendant's driving -- such as weaving or speeding -- is presented. Most other jurisdictions require additional evidence, according to Robert M. Goldstein, director of the D.C. Police Alcohol Countermeasures and Police Traffic Services.

In layman's terms, a blood alcohol level of .1 percent means that a person's body has absorbed so much alcohol that about one-thousandth of his blood volume is alcohol, according to the D.C. medical examiner.

The new law presumes that a person with a blood alcohol level of 0.05 percent is guilty of driving under the influence, a change designed to make it easier to prosecute, law enforcement officials say. In Maryland, the threshold for presumed driving under the influence is a blood alcohol level of 0.08 percent, and in Virginia it is 0.1 percent.

Any driver in D.C. who refuses to take a breath or blood test to determine his level of sobriety can lose his license for one year -- up from six months under the old law. Judges or juries may be told during a trial that the defendant refused to take the test, something not allowed under the old law here or under existing laws in Virginia and Maryland, according to Goldstein.

Goldstein said that more than one-quarter of the estimated 3,600 persons arrested here each year for drunk driving refuse to take the test, and prosecutors say prosecution has been difficult in those cases because juries often voted to acquit when no test results were presented.

The new law also imposes stiffer fines for repeat offenses. Anyone convicted of a second offense within 15 years of the first offense can be fined $5,000 and jailed for one year. A third offense within 15 years can bring a fine of up $10,000 in addition to a year in jail.

Anyone convicted of driving after his license is suspended or revoked now faces a maximum fine of $5,000, up from $500.

The new law reduces penalties for first offenders, but law enforcement officials say its practical effect will be more and speedier convictions. Under the old law, first offenders could be fined $500 and sentenced to six months in jail. After midnight tonight, first offenders can be fined $300 and jailed for 90 days. But, because the jail term does not exceed 90 days, first offenders will not be able to demand a jury trial. Instead, they could be tried before a judge, generally a much speedier process.