WASHINGTON (AP) _ Responding to President Reagan's call for a drug-free workplace, the National-American Wholesale Grocers Association on Thursday issued a how-to guide for setting up employee testing programs.

The guide is intended for the association's 400 members, who employ about 300,000 people at 700 distribution centers nationwide, but it has wide applicability to private industry generally, said John R. Block, association president.

''Number one, it's a safety question. ... It's a response to the president's call for a drug-free workplace and, there's no question about it, ... of importance to us is increased productivity,'' said Block, who took over as association president in February after stepping down as Reagan's agriculture secretary.

Reagan, in a televised address on Sept. 14 with his wife, Nancy, called for a national effort to eliminate drugs in the workplace. Block said White House aides helped the association formulate its model plan.

Block said drug and alcohol abuse is probably no worse among grocery industry workers than among workers in any other business. But, noting that working drug-free is critically important for heavy equipment operators and truck drivers, he said association members ''time and time again'' expressed concern about the problem.

Block said he has asked Labor Secretary William Brock and Transportation Secretary Elizabeth Dole to issue rules requiring heavy equipment operators and interstate truck drivers to submit to drug tests. He has not received a reply, but said Brock in a telephone conversation was receptive.

Block said the association's plan ''stresses fairness to the worker'' and includes guidance for setting up confidential employee assistance programs, but he acknowledged that the association was not recommending how extensive or expensive the assistance program should be.

He also said if an employee ''refuses to correct his shortcomings, he's going to be fired. I think we all know that.''

Peter A. Susser, the association's legal counsel, noted that federal employee unions are challenging drug testing as a violation of the Constitution. But, he said, the Fourth Amendment provisions concerning search and seizure do not apply to private employers.

The association plans two seminars on the model drug testing plan, Dec. 3 in Washington and Jan. 13 in St. Louis.

Before the first seminar, Block said the association's five officers will take urine tests. The group's 38 employees will not be required to take the test, but job applicants will have to submit to testing before hiring.