Defense lawyers battling to keep convicted murderer James Arthur (Slim) Calhoun out of the gas chamber lost a round yesterday when a Montgomery County Circuit Court judge ruled that the state's death penalty is constitutional.

That ruling by Judge William Cave now leaves it to a 12-member jury to decide in a hearing beginning today whether Calhoun will be sentenced to death for murdering county police officer Philip C. Metz during a robbery last March at a W. Bell catalogue discount store in White Oak.

Cave rejected arguments from defense lawyers yesterday that Maryland's death penalty is unconstitutional because it leaves local prosecutors some discretion in deciding when to seek the penalty for convicted murderers. The judge said that even if that contention were true, "there has been no showing that he Calhoun has been constitutionally discriminated against."

No one has been executed in Maryland since Acting Gov. Blair Lee III signed the death penalty statute into law in 1978, although three men and one woman are currently sentenced to die in the state's gas chamber. No Montgomery county jury has imposed the death penalty since the 1978 law gave the sentencing powers in such cases to juries instead of judges.

Defense lawyers succeeded on one point yesterday, however, when Cave ruled that Calhoun would not be required to submit to a psychiatric examination requested by the prosecution. Calhoun's attorneys objected to the examination, and the judge upheld their position.