SOMETIMES the federal Department of Agriculture can be tough on connoisseurs. Its rules, set to protect the health of Americans and American crops, often mean that specialty foods cannot be sold unless strict conditions are met.

The Sichuan peppercorn is a case in point. Since 1968 it has been illegal to import this spice, the dried berry of the prickly ash tree, because it might pose a threat to the American citrus industry. The trees can harbor a canker, harmless to humans but contagious among plants and for which there is no known cure. It has not been proven that the dried berries can transmit the disease but it is so devastating that about three years ago the Department began enforcing the ban in earnest.

Despite the ban, Sichuan peppercorns, an essential ingredient in many Sichuan dishes, were still being sold as recently as about a year ago, especially in Chinatowns. Then last year the department approved a treatment to destroy the canker, heating the berries to at least 140 degrees for 20 minutes.

Now, many spice dealers, including Adriana's Caravan in Grand Central Terminal, and several shops in Chinatown, sell Sichuan peppercorns in bags bearing labels stating that they have been heat-treated. Some stores in Chinatown sell them without the label.