An Upper Marlboro veterinarian pulled a severed cat's leg out of a paper bag today and demanded that members of the House Environmental Matters Committee "look at it" to emphasize the need for a law that would outlaw steel leg-hold traps in Maryland's urban areas.

Dr. John Hayes, who said he had never before taken a position on the issue, which annually is debated in a show-and-tell atmosphere in the Maryland legislature, said he removed the leg from a cat brought to his office last Saturday.

Hayes then plopped the trap down on the witness table and from a blue and white carboard box produced the live three-legged gray cat. "This creature was cut up by some idiot who set this trap," Hayes screamed.

Hayes demanded, and the committee acquiesed, that the severed leg encased in transparent plastic, be passed around the U-shaped table for a close-up inspection.

When Hayes took his act across the street to the Senate, where the Judicial Proceedings Committee was hearing an identical proposal, Chairman J. Joseph Curran (D-Baltimore) refused to allow Hayes to pass the leg around. The mere sight of the furry limb cause Sen. Margaret C. Schweinhaut (D-Montgomery) to gasp and cover her face.

The bill, sponsored in the House by Del. Andrew J. Burns (D-Baltimore) and in the Senate by Curran, would prohibit the sale or use of steel-jaw, leg-hold traps in Baltimore and Prince George's, Montgomery, Baltimore, Anne Arundel, Howard, Carroll and Harford counties.

Opponents of the measure, who last year spoofed the proponents by arousing their sympathy for "the Polish blue fox," a nonexistent animal, offered their share of the dramatics today also.

Clarence A. Leggett, a plaid-shirted trapper from Washington County, stuck his hand in a loaded trap to demonstrate that its punch is nonlethal. When a proponent suggested that Leggett had used a undersized trap to make his point, he replied, "I sure ain't gonna stick my fingers in a bunch of traps that have been hopped up by the humane society."

Ray Mosca, vice president of the Maryland Trappers Association, said trappers are "nature's veterinarians," and accused the proponents of "living in a Walt Disney world," instead of the "real world, where Bambi doesn't talk, thumper doesn't thump and Petunia the skunk don't smell so good."

Mosca ridiculed the proponents, saying "90 per cent who support the bill are women," until the four women members of the House committee scolded him, whereupon he told Del. Patricia Aiken (D-Anne Arundel), "honey, I love "em, God bless "em."

The leadoff witness for the opponents in both Houses was Sen. Frederick C. Malkus Jr. (D-Eastern Shore), who suggested that the proponents were following a "divide and conquer" scheme in which they propose to ban the traps in only seven central Maryland counties and Baltimore this year, and then return next year with amendments to extend the ban statewide.

"I recall from my days in the House," boomed the whitehaired senator from Dorchester County, "that the Humane Society was opposed to using dogs for medical experimentation," and instead suggested experiments be conducted on humans.

A Prince George's County physician, Dr. John Beary, said he recently treated a 4-year-old girl who caught her right hand in a trap that had been set near her home. The child underwent surgery and likely will "always have some impairment" of her hand, Beary said.

Because there are "two dramatically opposed sides" locked in an annual battle on the question, Del. Judith C. Toth (D-Montgomery) said she has introduced a compromise bill that would permit the use of traps only by farmers and their agents on their own land. Her bill would affect only Montgomery County, but she invited other delegates to add their counties to her bill.