Blockade flea and tick repellent, introduced four years ago in versions for both dogs and cats, was heavily promoted and immediately successful. In six months, Hartz Mountain Corp. sold more than 5 million of the shiny black cans with the trigger applicators.

But Blockade from the beginning was also a source of controversy, with some aggrieved pet owners complaining the product was a little too powerful. Not only were the fleas and ticks killed, but the pet suffered as well.

In a letter to the Environmental Protection Agency on Dec. 14, 1987, Hartz acknowledged that it was being blamed for 366 pet deaths and 2,700 pet injuries, not to mention 56 "alleged unsubstantiated human injuries." It took Blockade off the market for more testing.

Two years ago, it was reintroduced. The formula remained the same, Hartz having concluded that it was safe. The complaints that had been received, the company said, were a matter of people linking problems with their pets to the tremendous media attention Blockade was receiving. The repellent, in other words, was a scapegoat.

The product is not only the same inside the can. The packaging has also remained identical -- a matter of some curiosity to the EPA.

At the height of the controversy, in September 1987, Hartz was encouraged by the EPA to put hang tags on its existing inventory of canisters. The instructions on these tags were much more explicit than on the can itself, warning that Blockade should be applied lightly, for instance, and not used at all on kittens, puppies, pregnant cats or sick pets.

It was supposed to be a temporary measure. "Normally, we would have had them relabel it," said George LaRocca, a product manager in the EPA's registration division. But the label was printed directly on the can, which made such a step impractical. By now, LaRocca said, "stores shouldn't be selling any product with a hang tag."

Yet if you go into a grocery store and seek out a canister of Blockade, it will still have a hang tag on it. What was supposed to be done on "an interim basis" has become permanent.

For William Buck, director of toxicology at the National Animal Poison Control Center, Hartz's action -- or non-action -- is a matter of considerable puzzlement. "I would think that as soon as possible they would have changed the label," he said. "From a safety and a public-relations standpoint, it would have been better."

The center has studied Blockade several times, finally concluding that problems could develop when it was used too frequently, in conjunction with other pesticides, or too much was applied. "This could have been cleared up by the hang tag, but the hang tag was often missing," said Buck. "It was either taken off in the store, or the pet owner didn't pay any attention to it."

Hartz, moreover, is paying the center to operate a special toll-free line. Pet-owners who experience poisoning trouble with Hartz products can get immediate help by dialing 800-345-4735. Buck puts the number of callers at two or three a week. Does that mean Blockade is trouble-free?

Not by this evidence. "Since Blockade still has the old label, the number never got out," said Buck. "It's not listed on any Hartz product that I know of."

Hartz said it would refer any poisoning calls received to the center, and that in any case the number of complaints since reintroduction has been "minimal." But Buck still finds the whole thing rather frustrating. "If an animal reacts to that product, and we get with the owner or the veterinarian very quickly -- within a few hours -- we can almost always save the animal. It's when it goes on for two or three days, and they can't find anyone to help them, that they've got problems."

Hartz is steadfast in its response that the product is not and never has been a danger.

"We have not seen any information which on specific examination definitely shows that Blockade caused the problem with an animal," said a company spokesman. "The symptoms which have been attributed to having been caused by Blockade are ubiquitous." These include vomiting, tremors, seizures and lack of coordination.

If Hartz had to do the whole product launch all over again, would it do anything differently? "Probably not," said the spokesman, who asked not to be named.

That includes using the tag information on the original label. "I don't think it's absolutely essential advice. ... Testing was done on the same animals that were considered to be at risk, and they did not show any adverse reactions," the spokesman said.

Meanwhile, back at the EPA, the agency received a check from Hartz for $45,000 on Dec. 27. The money reflected a settlement of charges that the company failed to report all complaints it was receiving about Blockade.

The money may seem small change to a huge corporation like Hartz, but EPA associate enforcement counsel for pesticides Michael J. Walker noted that, "This is the first time in the 18-year history of the Federal Insecticide, Fungicide and Rodenticide Act that we have ever sought to enforce the particular provision that requires reporting of health effects data."

The EPA's first indication from Hartz that pet-owners were asserting side effects was in a letter dated June 8, 1987. "A few complaints have reached our company alleging that some pet animals treated with Hartz Blockade ... may have experienced some reactions," it said.

Many of these reactions were occurring in Florida, which led that state's Department of Agriculture also to contact EPA. The agency then asked Hartz for certain reports. On June 29, Hartz sent summaries of 32 of them.

Two and a half months later, when a frustrated EPA sent investigators to Hartz offices, they found 88 complaints on file as of June 29. Said EPA lawyer Scott Garrison: "The only things Hartz gave us were the things they knew we already knew about. Hartz's failure to report took away the agency's ability to take immediate action."

Hartz has said in a statement that "it reported all required information in a timely manner," and that the $45,000 was paid "to avoid possible lengthy and costly legal actions." The consent agreement did not admit liability.

The EPA's disagreement with Hartz is more basic than the matter of reporting information. "Blockade may not have caused all these incidents," said lawyer Garrison. "Some may be misuse, some may be animals that were ill to begin with. But there were far too many others for Blockade not to have been a factor."

LaRocca, the EPA product manager, said that "from what I can tell, based on the number of incidents and the kinds of incidents, the product was over-applied to small animals. They would be the more sensitive for weight-to-surface ratio."

Compounding this problem, he said, was the fact that "the directions were not real clear. There wasn't any specific direction on how much to apply relative to the size of the animal, or the age of the animal."

Getting a repellent strong enough to kill the bugs without harming the pet is apparently no easy task. Said Buck, the poison control center toxicologist: "It's so hard to find a good flea product. We're having problems with all of them, to some extent."