The thoroughbred board of the Maryland Racing Commission, which last month banned the use of the controversial drugs Butazolidin and Lasix as of Jan. 1, reversed itself yesterday, permitting limited use of Lasix and delaying the effective date of the tougher rules until March 15.

Yesterday's unanimous vote, taken under threat of a boycott by Maryland trainers, modifies a ban on the two so-called therapeutic drugs that have been legal in Maryland since 1974.

The board also decided yesterday to continue to let the racing stewards decide drug-related cases. This was considered a victory for the horsemen, because proposed legislation that will come before the General Assembly next month calls for statutory penalities on bute and Lasix cases.

Butazolidin is a painkiller. Lasix is a diuretic that, according to opponents of the two therapeutic drugs, helps bleeding horses continue to race, but may mask the pressure of illegal stimulants and depressants.

Alan Foreman, the assistant state attorney general who acts as the racing commission's lawyer, said yesterday that the latest board action should not be considered as an about-face.

"The board is relaxing the ban to allow for regulations and guidelines (to be written) as to how the rule will be enforced, so everybody knows where they stand," Foreman said.

The board's action last month came under heavy criticism by leading trainers, who Monday night approved a boycott of the entry box if the ban went into effect when Bowie Race Course opens Jan. 1.

The horsemen's opposition centered on two issues: the use of the Lasix on known bleeders, which has been standard operating procedure nationwide; and quantitative guidelines on what determines a positive test for bute.

Butazolidin still is permitted in the training of horses.

Horsemen testifying at yesterday's meeting voiced concern about the consequences of tests that would show a trace of the drug still in a horse's system because the equine system metabolizes slowly.

The horsemen also were upset by remarks made by Commissioner Robert Furtick last week, suggesting that severe penalities should be imposed for using bute or Lasix; a one-year ban for the first offense, three years for the second and lifetime banishment for the third.

Furtick, who called for yesterday's special meeting, clarified his earlier remarks, saying that he was referring to penalities for use of stimulants and depressants and not for the two therapeutic drugs.

March 15 was picked as the new date for banning bute and limited use of Lasix because that is the date that the Pimlico meeting opens. The horsemen asked yesterday for a 120-day delay, because the National Association of Racing Commissioners meets April 1 and is expected to draw up a nationwide medication policy.

Foreman said the following guidelines will be used for "bleeders":

The horse must be observed bleeding on the racing strip by a state veterinarian. If the horse is shipped in from another state, it must be certified as a bleeder by a vet from that state's racing commission.

Vets must have a list of bleeders posted in the racing secretary's office. A first-time bleeder remains on the list for 21 days, a second-time bleeder for 60 days. A third-time bleeder will not be allowed to race.

A "five-hour rule" will be observed.A bleeder must be delivered to a detention barn at least five hours before post time on the day of a race. A licensed vet may give him Lasix up to five hours before the race.

At a meeting of the regional chapter of the Horsemen's Benevolent and Protective Association last night at Laurel, executive director Fendall Clagett told his members that it was their unanimity in the boycott vote Monday night that resulted in yesterday's board action. He also said that he did not think the horsemen could live with the five-hour rule, but was hopeful a solution will be worked out.