KEEP in mind when shopping wholesale that you must be flexible, not only because prices may K change from week to week, but because quantities available may vary. A wholesaler may not break up a case of canned juice, for instance, but may sell two bottles from a case that has already been opened.

And you will have to get acquainted with wholesale weights and measures. A case of oranges, for example, weighs about 40 pounds no matter what the size of the orange. The difference is how many come in the case--48 to 56, if the oranges are large; 72 to 88 if they are medium; 113 to 138 if they are small.

Unless a wholesaler operates a "storefront," as noted, call ahead to find out what to expect from the operation and to alert the wholesaler to expect you. The following establishments have reported that they will sell at wholesale prices. This does not include wholesalers who sell to the public at retail prices. DISTRICT

* A. Litteri Inc., 517 Morse St. NE. 544-0183. 8 a.m. to 5:30 p.m. Monday through Saturday; 8 a.m. to 3 p.m. Saturday. Italian canned goods, spices, pasta, meats and cheeses. Storefront.

* Bay State Beef Co., 1431 Okie St. NE. 269-1100. 8 a.m. to 1:30 p.m. Monday through Friday. Cash-and-carry bulk meat.

* Blue Ribbon Provision Co., 5646 Third St. NE. 526-4940. 10 a.m. to 4 p.m. Monday through Saturday. Bulk meat and poultry; also kosher products, deli meats and condiments, salads.

* Center Market Provision Co. Inc. 115 R St. SW. 488-7490. 5 a.m. to 1:30 p.m. Monday through Friday. Meat in bulk.

* Claxton Inc., 240 E St. SW. 554-9230. 6 a.m. to 2 p.m. Monday through Friday; 7 a.m. to 9:30 a.m. Saturday. Frozen and fresh seafood.

* Continental Baking Co., 2230 Sixth St. NW. 9 a.m. to 6 p.m. Monday through Friday. 462-0990. Manufacturers of Wonder Bread and Hostess baked goods. This is the main office and thrift shop; 13 other shops located throughout Virginia and Maryland.

* Dekelbaum & Sons, 417 Morse St. NE. 546-3011. 6 a.m. to 2 p.m. Monday through Friday, 6 a.m. to 11 a.m. Saturday. Individual cuts sold at retail prices; bulk meat sold at wholesale prices.

* Happy Valley Produce, 509 Morse St. NE. 544-5414. 8 a.m. to 5 p.m. Monday through Friday; 10 a.m. to 4 p.m. Saturday. Chinese goods and produce--rice, sauces, canned items. Storefront.

* Imperial Produce, 350 Morse St. NE. 544-3666. 7 a.m. to 1 p.m. Monday through Saturday. Full service produce.

* Keany Produce Co., 1276 5th St. NE. 546-4444. 3 a.m. to 1 p.m. Monday through Friday; 3 a.m. to noon Saturday. Full line of produce; also eggs, margarine and cheese.

* Kolker Poultry, 1251 Fourth St. NE. 544-8000. 6 a.m. to 2 p.m. Monday through Friday. Individual chickens or 5-pound minimum on parts.

* Kramer & Sons Provision Co., 515 Morse St. NE. 543-0046. 7 a.m. to 5 p.m. Monday, Tuesday, Thursday through Saturday; 7 a.m. to 2 p.m. Wednesday. Individual cuts sold at retail prices; bulk meat sold at wholesale prices.

* Mailley and Quality Produce, 3150 V St. NE. 832-6100. 7 a.m. to 2 p.m. Monday through Saturday. Full line of produce; also imported produce. Will deliver for orders over $100.

* Moctec Enterprises, 1956 Montana Ave. NE. 7 a.m. to 7 p.m. Monday through Thursday; 7 a.m. to 5 p.m. Friday. 635-0958. Manufacturers of fresh corn tortillas, chips, tacos, fresh masa and hominy. Storefront.

* New York Beef Co., 421 Morse St. NE. 543-3100. 7 a.m. to 4 p.m. Tuesday through Friday. Bulk meat.

* Potomac Butter & Egg, 220 E St. SW. 554-9200. 6 a.m. to 2:30 p.m. Monday through Friday; 8:30 a.m. to 12:30 p.m. Saturday. Storefront.

* Seneca Valley Produce, 1309 5th St. NE. 544-2400. 4 a.m. to 2 p.m. Monday through Friday. Full range of produce.

* Sol Salins Inc., 1325 5th St. NE. 547-7900. 2 a.m. to 1 p.m. Monday through Friday; 1 a.m. to noon Saturday. Full line of produce.

* Taylor & Sons Inc. 1273-A Fourth St. NE. 544-2255. 6 a.m. to 3 p.m. Monday through Friday; 7 a.m. to noon Saturday. All cuts and grades of meat. MARYLAND

* Bon Ton Food Products, 3801 37th Place, Brentwood. 779-8800. 8 p.m. to 5 p.m. Monday through Friday. Distributors of pretzels, potato chips. Storefront.

* Hills of Westchester, 3400 Windom Rd., Brentwood. 864-4421. 9 a.m. to 5 p.m. Monday through Friday. Bakers of petit fours, cookies, dietetic cookies and fruitcake.

* Middle East Bakery Inc., 11400 Old Baltimore Pike, Beltsville. 937-2484. 6:30 a.m. to 4 p.m. Monday through Friday. Bakers of whole-wheat and white pita bread.

* National Food Produce, 4506 Hamilton St., Hyattsville. 277-3100. 6:30 a.m. to 4 p.m. Monday through Friday. Italian products, specializing in pizza ingredients.

* New York Cheesecake, 12159 Nebel St., Rockville. 881-3410. 9 a.m. to 5:30 p.m. Monday through Friday; 10 a.m. to 4 p.m. Saturday. Bakers of cheesecake, cookies and pies.

* Nick's Italian Sausage, 10707 Indian Head Highway, Fort Washington. 292-1400. 7 a.m. to 4 p.m. Monday through Friday. Manufacturers of kielbasa, country, cheese and Italian sausages. 10-pound box minimum.

* Organic Farms, 10714 Hanna St., Beltsville. 595-5151. 8 a.m. to 4 p.m. Monday through Friday. Organic fresh produce, dried fruits, nuts, cheese, eggs.

* Ottenberg's Bakery, 5605 Sargent Rd., Hyattsville. 559-2633. 8 a.m. to 7 p.m. Monday through Saturday. Thrift shop of full line of manufacturer's breads and rolls. Storefront.

* Schmidt Baking Co. Inc., 1309 E. Gude Drive, Rockville. 340-8740. 8008 Cryden Way, Forestville. 420-2422. 9 a.m. to 5:30 p.m. Monday through Friday; 9 a.m. to 4 p.m. Saturday. Thrift shop of one-day-old bread, rolls, cakes and pies. Cheaper than wholesale. Not manufactured on premises. Storefront.

* Smelkinson Brothers, 7855 Rapahanook Ave., Jessup. 621-1281. Tuesday through Saturday 9 a.m. to 5:30 p.m. Dried goods, canned foods, frozen products, paper supplies, cheese and eggs in institutional sizes and single serving. Storefront.

* Stanley Foods & Equipment Co. Inc. 8545 Ashwood Drive, Capitol Heights. 336-4600. 8:30 a.m. to 4 p.m. Monday through Friday. Pizza products, frozen meats, deli condiments and meats, coffee and tea.

* Swiss Pastries, 2377 Lewis Ave., Rockville. 881-8158. 9 a.m. to 3:30 p.m. Tuesday through Sunday. Bakers of pies, coffee cake, danish, fancy cakes.

* Troisarc Gourmet Foods, 1106 Taft St., Rockville. 424-1494. 1:30 p.m. to 4 p.m. Monday through Friday. Manufacturers of deli and salad sandwiches on homemade rolls; salad or vegetable fillings in homemade whole-wheat pita; microwave sandwiches. Will deliver if en route. Minimum purchase of $30.

* Tuscarora Valley Beef Farm, 627 Sligo Ave., Silver Spring. 588-5220. Pennsylvania farmer who sells full line of beef and pork, lamb sold by the side, quarters of milk-fed veal, beef sausage, nitrite-free bacon and hot dogs. Will sell all quantities, from one pound to whole hinds. Must place orders by phone and customers come to "consumer pick-up" points throughout metropolitan area. Will deliver to family co-ops.

* U.S. Fish, 10576 Metropolitan Ave., Kensington. 933-9200. 10 a.m. to 7 p.m. Tuesday through Friday; 9 a.m. to 6 p.m. Saturday; noon to 6 p.m. Sunday; noon to 7 p.m. Monday. Wholesale prices for fish in bulk; retail prices for small quantities. Storefront. VIRGINIA

* Alexandria Seafood & Poultry Co., 203 E. Custis Ave., Alexandria. 836-0024. 6 a.m. to 3 p.m. Monday through Friday. Fish, poultry, cheese, eggs, shortenings, frozen vegetables in institutional packs.

* Casa Suarez in S & K Industries Inc., 871 Grace St., Herndon. 471-6219. 10 a.m. to 6 p.m. Monday through Friday; 10 a.m. to 2 p.m. Saturday. Manufacturers of fresh corn tortillas, tacos, chips and masa harina. Storefront.

* Crustie's Bakeries Inc., 2824-A Dorr Ave. Fairfax. 560-1636. 8 a.m. to 6 p.m. Monday through Friday; 9 a.m. to 5 p.m. Saturday; 2 p.m. to 5 p.m. 25 percent off on anything in store (not special orders). Full-service swiss bakery; cookies, cakes, croissants. All baking done on premises. Storefront.

* Randy's Meats & Seafood Inc., 9013 Centreville Rd., Manassas. (703) 361-5680. 10 a.m. to 6 p.m. Monday through Friday; 9 a.m. to 5 p.m. Saturday. Pre-cut, pre-packaged and frozen meats (hamburger patties, steaks, roasts) and full line of frozen seafood. Also institutional-size canned goods. Storefront.

* Washington Fish, 1200 First St., Alexandria. 549-4000. 5:30 a.m. to 5 p.m. Monday through Friday. Institutional-pack fish.