ONE USEFUL function of the stock market is to provide a running commentary on American states of mind. This week's performance is not entirely comic. On Tuesday night, a Florida investment adviser wired his large following of clients to sell everything. They did. On Wednesday, the volume of trading set records on both the New York and American exchanges. Prices fell heavily, and the fall continued yesterday.

Financial history is full of ripples of panic like this one. But it remains remarkable that one voice alone suddenly shouting "Sell" should set off a stampede in markets as large as New York's. There's a warning in this curious event, and it needs to be taken seriously by Mr. Reagan and the people who advise him on economic policy.

It's a warning that the markets are very nervous and inclined to wild overreaction. They are made up of people, after all, who are now dealing with inflation and interest rates far beyond any past experience. In this atmosphere, each new political rumor from Washington resonates like a plucked string. As they are being received in New York, the signals from Washington are uncomfortably ambiguous. The election of Mr. Reagan was taken to be a very good omen for financial stability, on grounds that it would mean tight budgets, tight money and less inflation. But now the people around Mr. Reagan are murmuring gloomily of a budget deficit this year as large as last year's -- or larger. None of the forecasts shows much progress toward lower inflation. Mr. Reagan certainly intends to cut taxes. But it's not clear that he means to reduce spending at the same time. What does that mean for future inflation? The people whose business is investment are now trying desperately to read these mixed signals.

Trouble in the stock market can impede the kind of investment and economic growth that mr. Reagan presumably wishes to encourage. But there is a more immediate threat, and that is the possibility that this ripple of panic will spread into the international currency markets. The dollar is floating along pretty steadily at the moment, mainly because of bad economic news from Europe and the weakness of the German mark. But currency exchange rates are no less sensitive to fears of future inflation than securities prices are. The message from the markets is that Mr. Reagan will not have much time to establish a clear line of policy on everything that touches inflation, taxes and money.