A pesticide banned for most uses since 1975 abruptly resurfaced in Arkansas this week, forcing thousands of gallons of milk products off grocery shelves and raising new questions about the effectiveness of food-safety regulations.

Two Arkansas dairies have voluntarily recalled more than 62,500 gallons of milk, and 38 Arkansas dairy farms have been quarantined since Sunday when federal officials found the milk contaminated with as much as seven times the acceptable level of heptachlor.

This once-popular pesticide was banned on most food crops in 1975 after being identified as a probable human carcinogen. Virtually all of its other uses on food or feed crops have been phased out and, for the last three years, the chemical's only legal use has been as a termite killer.

Federal and state officials are trying to determine how the chemical contaminated milk produced by Arkansas dairy farmers and shipped to Oklahoma and Missouri.

Dr. Tom Butler, an Arkansas Health Department official, said the heptachlor apparently came from feed supplied to the dairymen by Valley Feeds in Van Buren, Ark. Valley Feeds runs a gasohol plant, turning grain into fuel alcohol and reselling "mash" from the distillation process as animal feed.

Food and Drug Administration officials said they suspect that Valley Feeds used seed corn treated with heptachlor to protect against mildew and rot. Heptachlor was legal for that use until 1983.

Federal rules specifically forbade use of heptachlor-treated grain as animal feed because it accumulates in animal tissues, milk and eggs. The rule-writers apparently never contemplated the same grain being run through a gasohol plant and fed to animals.

Four years ago, a similar situation in Hawaii led to quarantine of virtually all dairy operations there. Heptachlor was legal for use on pineapple fields until 1982, and pineapple leaves were a popular feed for dairy cattle.

Two years earlier, the Agriculture Department was forced to scramble to retrieve more than 20 tons of heptachlor-tainted pork shipped to school lunch programs in Arkansas and Louisiana. USDA officials had tested the pork when packaged but did not analyze the results until six weeks later when more than seven tons had been consumed.

Arkansas officials, struggling to contain public fears and calm irate dairy farmers, acknowledged yesterday that they would not have detected the heptachlor-tainted milk themselves. The tipoff came from FDA inspectors conducting tests at Valley Feeds.

"We don't normally test for it," Butler said. "FDA caught it and came and told us. We immediately shut down our water lab and turned it over to heptachlor testing."

Arkansas officials said the milk does not pose an immediate health threat, although pregnant women and nursing mothers in northwest Arkansas, eastern Oklahoma and southern Missouri have been advised to substitute powdered milk.

Butler said the quarantine would stay on the dairy farms until heptachlor in milk returned to acceptable levels. "In Hawaii, that took six months," he said.