Starting today, America's first mass-produced gold medallions will go on sale in post offices across the country. The initial price is $646 for a one-ounce medal bearing the portrait of painter Grant Wood, and $323 for a half-ounce likeness of singer Marian Anderson, the Treasury announced yesterday.

The gold medallions, which were authorized by Congress and struck by the U.S. Mint, are intended to be this country's answer to the South African Krugerrand, an effort to keep U.S. gold at home. The general fund of the U.S. Treasury will benefit from the sales.

The start of the sale was delayed for a month because of difficulty of distributing official order blanks to all 35,000 post offices. The medallions will be sold only at post offices, and the quantity is limited to three of each size. Payment can be made only by certified check, cashier's check or postal money order.

Customers will receive acceptance notice of their orders within two to three weeks and actual deliver will take between eight and 12 weeks. If the medallions arrive damaged they will not be replaced. If they are lost in the mails, the Treasury will refund the purchase price. The sale will continue until Sept. 30 unless all the medallions are sold before then.

The purchase price changes daily and is based on the previous day's settlement price of spot gold on the New York Commodity Exchange plus a premium of $12 for the ounce medallion and $6 for the half ounce medal. Yesterday's closing price was $633. The Treasury rounded out the price for the smaller medal to $323 and doubled it for the larger. To ascertain the price on a given day, Washington buyers should telephone 783-3800 for a recorded message. Outside this area the number is (800) 368-5510.

Though gold medallions have been minted before by the U.S. government, this is the largest striking. About 36,000 were produced for the Bicentennial. One million of the half-ounce size and 500,000 of the ounce size will be offered. The Treasury plans to offer one-ounce and one-half-ounce medallions bearing images of other famous Americans each year through 1984.

Unlike the one ounce Krugerrand -- which was selling yesterday at Deak & Co. for $671 -- the U.S. gold medallion is not a coin, not legal tender. The Treasury will not buy back the medallions. To sell, it suggests contacting a collector or dealer. Stella Hackel, director of the U.S. Mint, said yesterday she knows no dealers that plan to make a market in medallions. A Deak representative said yesterday his company had no current plans to trade medallions but might reassess the situation six months from now.