Consumer prices in the Washington area declined 0.2 percent during the last two months, primarily because of lower apparel and private transportation costs, the Labor Department said yesterday.

The decrease during December and January follows consumer price increases of 0.7 percent in October-November and 1.5 percent in August and September. The index is calculated every other month.

For the 12-month period ending in January, Washington-area prices rose 3.6 percent.

The Washington-area consumer price index now stands at 314.6, which means that an item that cost $10 in 1967 now costs $31.46.

Seasonal markdowns for all types of clothing and footwear caused the prices of apparel and upkeep to decline 3.5 percent, the Labor Department's Bureau of Labor Statistics said. Those prices had declined 1 percent in October-November, after rising 6.2 percent in August-September.

Transportation costs decreased 0.5 percent because of the continued drop in gasoline prices and a slight decline in car prices, the department said. Transportation costs had increased 0.2 percent in the two months ended in November and had been unchanged in the previous period.

The cost of housing in the Washington area was unchanged on average in the last two months, as the lower costs of house furnishings and household fuels offset higher renters' and homeowners' costs, the department said. The Washington-area prices of food and beverages, entertainment and other goods and services all rose by less than 1 percent.

The Labor Department's index of local medical-care prices declined 0.3 percent in the last two months, compared with increases of 1.9 percent in the two months ended in November and 1.2 percent in the two months ended in September.

However, officials of Blue Cross and Blue Shield of the National Capital Area said they believe the Labor Department's method underestimates health costs. The insurance provider has developed its own measure of local health-care prices, which shows local costs generally higher -- but declining more sharply in recent months -- than the department estimated.

The Blue Cross and Blue Shield study said per capita average health costs declined 4.1 percent in the three months ended in September, compared with the previous quarter. However, these results represented a 0.8 percent increase over the cost of local health-care services in the same period of 1983.

The Labor Department bases its medical care index on the price of a hypothetical "basket" of commodities and services, which include prescription and non-prescription drugs, wheelchairs, eyeglasses, health insurance, hospital charges and dentists' and physicians' fees.

Blue Cross and Blue Shield, using data from the paid-claims files of almost 600,000 local non-federal subscribers, measures the total average charge per person by including changes in prices, utilization and mix of health-care services.

Thus, if local prices remain unchanged but hospital use drops, the average cost of health care declines. Blue Cross includes all medical services performed, rather than a sample of services. However, the insurance group's index does not include the costs of prescription and non-prescription drugs, eyeglasses or dental care.