With Sen. Jesse Helms (R-N.C.), chairman of the Senate Foreign Relations subcommittee on the Western hemisphere, scheduled to hold another hearing on Mexico today it is appropriate to recall what happened at and after the hearing he held last month. He gave a forum to an official, Customs Commissioner William von Raab, whose expressions were so inflammatory that no less an administration figure than Attorney General Edwin Meese felt compelled to apologize for and disavow the commissioner's views. Sen. Helms, however, was unrepentant. Of Mexican protests about some of the allegations loosed at his hearing, the senator declared, according to the Charlotte Observer: ''All Latins are rolatile people. Hence I was not surprised at the volatile reaction.''

These are extraordinary wordsextraordinarly false, condescending and harmful words. It is offensive not simply to categorize Latins as volatile, a word that has the effect of dismissing the legitimacy of their reactions, but to lump ''all Latins'' in a single categoryany single category. Sen. Helms may be unconcerned, but it remains an undeniable embarrassment to American foreign policy that an American legislator who purports to exercise oversight of the conduct of American relations with a whole hemisphere can blithely put several hundred million disparate people down. How is it to be explained to Latin Americans that the practice of ethnic insult is still a live political commodity in the United States?

Relations between Mexico and the United States may never have been so central and so delicate. Mexico is going through a painfully demanding phase brought on by a thoroughly commendable effort to continue its modernization program, notwithstanding circumstances of savage difficulty. This effort brings it into tension with the United States in a variety of ways. For the benefit of both countries, however, it is essential that the resulting strains be treated in a mature and statesmanlike way. there is plenty of room for argument and for criticism of Mexican policy without the sort of gratuitous provocation that merely raises hackles in Mexico and spreads doubts in both countries about whether the United States has the steadiness the relationship requires.