The Mexican Peso has been falling erratically, signaling renewed troubles in the country's economy. The immediate causes are rising inflation and capital flight. Behind that looms the price of oil and the effect of its dropju on a country that had become crucially dependent on it.

Mexicans are reportedly again discussing some sort of limit or moratorium on payments of interest on their foreign debts. That could provide a measure of relief in the very short term, although it would not be likely to last long. The greatest strain is not in Mexico's foreign payments, but in its domestic budget. The decrease in oil prices has cut budget revenues by one-fourth since the beginning of the year. Mexico's deficit is now, in proportion to its economy, more than twice as large as the United States' deficit -- and is even harder to cut, for Mexico is now falling into another severe recession.

Mexico will need new loans again this year, and the negotiations are proving unusually difficult. The International Monetary Fund won't provide more money until the Mexicans agree to reduce their budget deficit further. And until the IMF begins to lend again, the commercial banks are reluctant to provide more money.

The Reagan administration, recognizing the dire character of Mexico's circumstances, is evidently trying to put an end to the recent spate of inflammatory and inaccurate attacks on Mexico. David C. Mulford, assistant secretary of the Treasury for international affairs, observed yesterday that the Mexican government has already taken a number of important and, in political terms, unpleasant measures to strengthen the economy. "These difficult steps have not been given the recognition they deserve," he said. He was testifying before Sen. Jesse Helms's subcommittee -- the same one that last month elicited the wild testimony from other administration officials about drugs and corruption in Mexico. The Treasury appears to have decided not to assist Sen. Helms in his vendetta against Mexico.

Mexico's boom ended abruptly four years ago, when it ran out of money to meet its debts abroad, and since then Mexicans have been through a harsh period of recession and adjustment. By last year, diligence and patience seemed to be paying off with a return to growth and stability. Then oil prices broke. "Coming as it did after four years of economic adjustment," Mr. Mulford said, "this traumatic contraction was not only economically painful but politically demoralizing."

Mr. Mulford was letting the Mexicans know that their exertions do not pass unnoticed. Unfortunately, there is still a great deal for them to do, none of it easy. But the implication is that, as long as they continue to work in good faith, they can expect support from the United States