A Prince George's County Circuit Court Judge has dismissed all charges -- including first-degree murder -- against a 27-year-old Washington woman because county prosecutors failed to bring her case to trial within the required 180-day period.

The Maryland attorney general's office believes it is the first case involving a murder suspect to be dismissed under Maryland's new speedy trial rule, which requires defendants to be brought to trial within six months of their arraignment.

The suspect, Gloria Lee Mason, 27, had allegedly participated with two other persons in the shooting death last July of a Hillcrest Heights man, and the robbery of his son. The nine charges against Mason -- who pleaded innocent by reason of insanity -- included murder, armed robbery, assault, theft, use of a handgun, and other related charges.

Mason's attorney, Edward Smith Jr. said her reaction on March 31 when Circuit Court Judge William H. McCullough dismissed the case was total disbelief. "She was dazed that she didn't have to go through a trial," he said. "This was a big case. They could have asked for life or the death penalty."

County prosecutors blame the delay in bringing Mason to trial on a state mental hospital evaluation of her that was not completed in time and an overloaded trial schedule. As a result, the case was continued numerous times, until more than 200 days passed from the time of Mason's initial appearance in court.

The 180-day deadline, called the "Hicks Rule," was established last year by the Court of Appeals in order to guarantee defendants their constitutional right to a speedy trial.

Betty Ryon, the daughter of Harry Lee Spalding Sr., the murder victim, said she was shocked by the decision to dismiss charges against Mason. "Justice stinks. Trying these cases in 180 days is fine with me," she said, "but I think there should be exception when the courtroom is overbooked."

Assistant State's Attorney William T. Shockley said he sympathized with Ryon's feelings, and indicated that prosecutors would probably appeal Judge McCullough's decision to the Maryland Court of Special Appeals. The appeal would probably be based on the delay by the state mental hospital in making its evaluation of Mason.

Exceptions to the 180-day rule are permitted for "extraordinary cause." But that term never has been defined in Maryland's courts, according to Deborah Handel, chief ot the criminal appeals division of the Maryland attorney general's office.

Handel said that it is unlikely the charges against Mason could again be brought against her, because that would defeat the purpose of the speedy trial rule.

However, Mason could be charged with appropriate new charges, if ones could be found, Handel said. County prosecutors said yesterday that they were investigating that possibility.

Spalding, the murder victim, was a 67-year-old retired electrica worker. On the night of July 19, 1979, police said, intruders forced their way into Spalding's home in Hillcrest Heights, robbed his son Harry Lee Jr., and shot and killed the elder Spalding.

The other two suspects charged in the case, prosecutors said, were David Hinson and Ronald Johnson. Johnson is still at large. Hinson is part of a plea bargain to obtain his testimony against Mason, pleaded guilty to a reduced charge of second-degree murder and received a 30-year prison sentence, with 10 years suspended.

Spalding's daughter says she intends to make sure that if Johnson is caught, he is brought to trial within the required 180-day period.

"I'll remind them (the prosecutors) every day that this is day 177, this is day 178, and this is day 179. I don't want them to say that this just slipped through," she said.