Nearly 28.5 percent of this country's personal wealth is in the hands of 3.3 million persons whose holdings exceed the entire U.S. gross national product, according to the Internal Revenue Service.

The IRS says the richest 1.6 percent of U.S. adults -- those with assets of $500,000 or more -- had total holdings of $4.3 trillion and a combined net worth of $3.8 trillion in 1986, the latest year for which figures are available. By comparison, the value of all goods and services produced in the United States in 1986 -- the GNP -- was $4.1 trillion.

Four years earlier, 23 percent of personal wealth in the United States was held by 2.2 million people with a net worth of $2.1 trillion.

The IRS report suggests the United States has well over 1 million millionaires. In 1986, the IRS estimated, there were 941,000 adults with a net worth of $1 million or more, almost twice the 475,000 reported in 1982 and five times the 180,000 in 1976.

The report, based on estate tax returns, tracks other recent findings that the number of wealthy people and their share of the pie grew sharply over the past decade. Those studies attributed the increase to an economy that grew for seven straight years and to major changes in tax law.

The IRS study estimates the 3.3 million wealthiest Americans had an average net worth in 1986 of $1.13 million. The portion of women among the super-rich continued to grow; they accounted for nearly 43 percent of total assets and on average owed less and had a greater net worth than men.

Past studies found the wealthy putting the biggest portion of their money into real estate, but now the No. 1 investment choice is corporate stock.

''Considering the relative performance of the real estate and stock markets between 1982 and 1986, this reversal was not unexpected,'' wrote IRS analysts Marvin Schwartz and Barry Johnson.

''The Dow Jones industrial average more than doubled during this period, rising 103 percent, while the price index of new one-family houses sold rose a comparatively modest 14 percent,'' the report noted.

More than one-quarter of the richest adults were in California (558,000) and New York (340,000). Texas, battered by declining oil prices, dropped from second to third place with 250,000; Florida had 238,000; Illinois, 148,500.

On a per capita basis, the IRS said, Connecticut had the largest concentration of wealthy people, at 327 per 10,000 adults. California had 299 per 10,000. The national average was 198 per 10,000.

For statistical reasons, the IRS did not count the 25,100 people with net worths of $10 million or more when estimating the wealthy in each state, although they are included among the 3.3 million. They were worth nearly $538 billion, an average of $21.4 million each.

Average net worth of the rich ranged from $473,510 in North Dakota to $1.25 million in Oregon.

The findings are reported in the latest IRS ''Statistics of Income Bulletin.''