Fairfax County prosecutors yesterday challenged a lower-court judge's belief that the state's drunken driving law is unconstitutional, arguing that the statute does not put motorists in the position of having to disprove intoxication.

The arguments came in two DWI cases as defense attorneys -- who agree with General District Court Judge Ian M. O'Flaherty's opinion that the law is unconstitutional -- asked a Circuit Court judge to dismiss the charges. Fairfax Circuit Court Judge Robert W. Wooldridge Jr. set both cases for trial in December and said he would issue his rulings before then.

Whichever way Wooldridge rules, the cases are almost certainly headed for the Virginia Supreme Court -- and a ruling on the legality of the state's laws on driving while intoxicated.

Both DWI cases originated in the lower court, where O'Flaherty had made it clear in other cases that he believed the state's DWI laws made some unconstitutional presumptions. The law states that drivers with a blood alcohol level of .08 are presumed to have been drunk at the time of the alleged offense.

Although he has relied on the law to convict drunk drivers for many years, the judge also has a problem with presuming that a person's blood alcohol level 90 minutes or more after the arrest, when most breath tests are given, is the same as it was at the time of the arrest. The law says both presumptions are "rebuttable," meaning defendants can offer their own evidence to contest police evidence.

But O'Flaherty, citing a 1985 U.S. Supreme Court ruling, said requiring a defendant to do anything in his own defense violates the Fifth Amendment. The government is required to prove criminal cases beyond a reasonable doubt; the defendant is not required to do anything, the judge said. Twice, he has declared the state law unconstitutional.

Virginia prosecutors may not appeal constitutional rulings, or anything else, from General District Court. And it was only a pair of state constitution changes, approved by voters more than a decade ago, that allowed prosecutors to appeal from circuit courts, and then only on constitutional and evidentiary issues.

So Fairfax prosecutors began asking O'Flaherty to dismiss drunken driving cases that landed in his courtroom. There are 10 General District Court judges in Fairfax, and as many as five are hearing randomly assigned traffic cases any one morning. When O'Flaherty dismissed a case, prosecutors then indicted the defendant in Circuit Court, where either side could appeal.

This month, O'Flaherty began refusing to dismiss cases for possible indictment, saying he had done so enough times for appeal purposes. He then began finding some defendants not guilty, in part by refusing to presume that they were drunk based on their blood alcohol test.

Corinne J. Magee, representing Edward Q. Yap in one of the cases before Wooldridge yesterday, said a law telling a judge or jury to presume someone was drunk, based solely on their blood alcohol level, "puts the onus on the defendant to disprove his own intoxication." She said such a defense "likely will not be found sufficient."

Assistant Commonwealth's Attorney Casey M. Lingan said the law's presumption of drunkenness does not shift the burden to a defendant but merely allows "a permissible inference that's based on loads and loads of scientific study." He said the presumption "takes away the necessity of having an expert hauled into court every time a misdemeanor [drunken driving] case is held."

Todd F. Sanders, representing the second defendant, Gonzalo Padilla, said: "This is not an inference. It's a presumption under the statute and the way that every judge in this circuit interprets it." He said defendants could combat the blood alcohol presumption only by calling their own experts to testify.

Lingan also argued in a brief that the Supreme Court case cited by O'Flaherty, Francis v. Franklin, dealt with the intent of a murder defendant and an instruction that told jurors to presume that a suspect had intent to kill if he was "of sound mind." Lingan said "intoxication presumptions are based on scientific research. . . . Thus the presumptions of intoxication are inherently reliable as opposed to subjective like those in" the Francis case.

Assistant Commonwealth's Attorney Kathryn S. Swart, who has argued so vigorously with O'Flaherty that he threatened her with jail, said defense attorneys can rebut the presumption by cross-examining police officers and attacking other prosecution evidence.

In a brief, Swart wrote that the Constitution "does not prohibit the use of presumptions or inferences as procedural devices to shift to the accused the burden of producing some evidence contesting the otherwise presumed or inferred fact."