'Dear Mr. Regan: If inflation is easing, why don't I notice it?'

''I begin to wonder,'' says a letter writer, ''if you people in Washington have any idea of what is going on out here in the real world?'' Inflation - or rather what he regards as the Reagan administration's one-sided outlook on inflation - has this Oklahoma writer up in arms.

''I have just completed paying my first-of-the-month bills,'' the writer continues, ''and thought you should know that we have not in any way seen any of this declining inflation you keep talking about.''

The ''you'' to whom the letter refers is Treasury Secretary Donald T. Regan, point man for the administration's claim that inflation is on the run.

A Pennsylvania man puts the same question another way. ''Why have my wife and I not benefited from the easing of inflation, for which the administration takes credit?'' he asks.

''Our apartment rent was recently increased by 20 percent. Our nine-year-old car needs parts and repairs, the cost of which are up substantially. Our newspaper is up 25 percent. Though we are light eaters, our food costs are escalating. Medical insurance costs keep rising.''

Administration officials do indeed take credit for bringing inflation under control. They stress that so far this year, consumer prices have risen at a 3.7 percent annual clip, compared with 3.9 percent for all of 1982, 8.9 percent in 1981, and 12.4 percent in 1980.

Millions of Americans know about the figures. But they detect little improvement in their own situations.

''Is there a relationship between inflation and the cost of living?'' the Pennsylvanian asks.

A clue to the answer lies in words used by the letter writers themselves. The Oklahoman speaks of ''declining inflation.'' The Pennsylvanian cites the ''easing of inflation.''

Both phrases refer to the economic situation in which the United States now finds itself. The rate of inflation is declining, but prices continue to climb. Some inflation, in other words, persists. This translates into higher price tags for almost everything a consumer buys.

Deflation, when price levels actually decline, is something else - a condition the US has experienced only twice since World War II, when the consumer price index registered an overall decline in 1949 and 1954.

In two ways the burden of inflation falls disproportionately on elderly Americans with fixed incomes - a group to which both letter writers belong - and other low-income people.

First, their incomes fail to rise commensurate with inflation. It is true that social security benefits have been linked to the consumer price index (CPI). Checks dated Jan. 3, 1984, for example, will include a 3.5 percent cost-of-living increase, based on CPI performance.

But a broader measure of inflation, called the GNP deflator - which tracks all prices - is thought to be running at about a 5 percent annual rate. The 3.5 percent cost-of-living adjustment will not keep social security beneficiaries fully abreast of rising costs.

Second, elderly and low-income Americans spend a major share of their income on goods and services such as medical costs, food, shelter, utilities, and clothing. Prices in these categories often rise faster than the CPI as a whole.

''Barring the unforeseen,'' a senior Federal Reserve official says, ''renewed inflation is unlikely to be an early danger.

''Oil prices, which were the major culprit in the 1970s, should not rise soon - given conservation and growing non-0PEC (Organization of Petroleum Exporting Countries) supplies.''

''Under these circumstances, inflation will come primarily from classical excess demand - too much money chasing too few goods,'' the central banker explains.

''This is not likely to occur, because interest rates at their current (high) level will dampen growth somewhat.''

Secretary Regan is correct when he heralds the gains made against inflation.

But the writer from Pennsylvania, describing his wife and himself as in their late 70s, also is right from his own point of view:

''Our many contemporaries in the same circumstances fail to understand why they are not benefiting from the easing of inflation.''