Shortly after noon yesterday, sisters Heather and Debbie Chirtea of Montgomery County embarked on their traditional day-after-Christmas shopping spree with $1,100 in Christmas gift money in their pockets. The idea, explained Heather, was to take advantage of the sales prices and stretch their money.

"We're looking for a lot of new clothes and other stuff," said Heather, 19, a student at Montgomery College in Rockville.

In two hours of shopping at White Flint Mall in Bethesda, Heather and Debbie, 21, each bought a pair of red slacks. Heather was wearing a newly purchased pair of black boots and was thinking of buying a black leather jacket from Bloomingdale's that had been priced at $160 but was on sale for $100. While contemplating the purchase, she moved on to Lord & Taylor's to browse through the sweater department.

"It would be so easy to spend it all today," sighed Heather. "But I'm bargain hunting."

Thousands of other shoppers on the lookout for after-Christmas bargains crowded shopping malls in Montgomery County and northern Virginia and in the District of Columbia yesterday, during one of the busiest retail days of the year. Generally, according to merchants, the day after Christmas ranks in sales volume only behind the two Saturdays before Christmas, Christmas Eve, and the Friday after Thanksgiving.

Yesterday's hardy holiday shoppers endured 20-degree weather, traffic snarls and long lines at cash registers to take advantage of reductions of up to 50 percent on winter clothing, linens, glassware, and on Christmas cards, ornaments and wrapping paper. Retailers said that, on average, it was a successful shopping day, with several stores enjoying increases in sales over the same day last year.

"The store is jumping," said Gary Kaufman, vice president and store manager of Bloomingdale's at White Flint Mall. He said the store's traffic was up 10 to 15 percent over Dec. 26, 1984.

"It looks to be a banner day," Kaufman said, adding that the store's juniors' clothing and Christmas trimmings and wrap departments were the busiest.

Jim Wells, vice president for marketing for the 16 Woodward & Lothrop stores in the Washington area, said that by 4 p.m. the stores had exceeded last year's sales for the same day.

"I'm very pleased," he said. Wells added that Christmas trimmings, gloves and jewelry all had drawn large crowds.

Although this area's Sears stores had "good traffic" yesterday, they were no more crowded than last year, said Mary Strahlendorf, spokeswoman for the Sears Roebuck and Co. eastern region.

"We are expecting Saturday to be busier because of the fact that Christmas fell on a Wednesday and a number of people had to go back to work today," she explained.

Many who braved the crowds did so with a purpose, be it exchanging ill-fitting garments or stocking up on discounted gifts for next year. And at least one bewildered shopper was overheard to exclaim: "It's a zoo in here."

Ken Corbin and his daughters, Shira, 8, and Dara, 4, who wore matching fur coats that had been Christmas presents from their grandmother, picked through the rolls and rolls of half-priced Christmas wrapping paper offered at Bloomingdale's.

"We pick out the more neutral ones, so we can use them throughout the year," Corbin explained, while instructing Dara on the most suitable colors for all-purpose occasions.

"That's a good one," he said. "Mom will like that."

Barbara Plummer and her daughter Amy spent $60 on hand-blown, hand-painted Italian glass ornaments, a wreath and Christmas cards.

"Lord & Taylor's is one of the few places you can find these ornaments," said Plummer, who works as a sales clerk in a store downtown. She added that she already has more than 1,000 of the ornaments, but likes to wait until the after-Christmas sales to buy still more at 50 percent savings.

Elizabeth Logan of Annapolis and her married daughter from Connecticut opted to exchange a coat yesterday that Logan had given another daughter, Tracey, for Christmas.

They said that they had telephoned various Lord & Taylor's stores, but the only one to have a coat in the right size was at White Flint, so they decided to spend the day at the mall and also visit the Talbots and I Magnin stores.

"We left the men at home taking naps, putting away the Christmas toys and reading the boat sales," Logan said, "and we came here to do what we girls like to do: shop."