Pacheco is internationally known for spearheading The Silver Spring Monkeys case - part of a landmark 15 year battle to rescue a band of high profile primates who had been crippled in laboratory experiments.

Pacheco recruited Republican Congressman Robert Smith of New Hampshire who introduced federal legislation designed to force the federal government to release these mutilated animals; legislation which was co-sponsored by over 200 Members of Congress, though 'killed' each year by Democratic Congressman Henry Waxman of California.

At the same time Pacheco was the central plaintiff in civil litigation against the federal government over custody of the animals, litigation which received hearings before the U.S. Supreme Court - twice - winning one and losing one.

Laboratory experiments on animals began in earnest in the 1950's with thousands upon thousands of laboratories in operation today, employing hundreds of thousands of individual experimenters, using in excess of an estimated 100 million animals annually in experiments that involve killing the animals after they have been experimented upon.  

Pacheco worked undercover in the federally funded Institute for Behavioral Research and for the first time in history, a laboratory was shut down because of cruelty to animals.

Pacheco performed grisly undercover work posing as a research assistant and covertly documenting the suffering of the animals after they had been intentionally crippled.  

He clandestinely brought in medical experts, produced sworn affidavits from those experts, and lobbied Congress to pressure law enforcement officials (over the vehement objections of federal agencies, universities and biomedical associations) to carry out the world's first police raid on a research laboratory because of cruelty, and executing a search and seizure warrant.  

This produced the world's first and only police confiscation of laboratory animals. Followed by the first arrest warrant, criminal trial, and conviction of a research scientist on cruelty charges, spawning a series of front page political and legal events that is credited by many with launching the U.S. animal rights movement.

In the process producing many landmarks such as:
    •    The 1st time an experimenter had ever been arrested for cruelty.
    •    The 1st time an experimenter had ever been prosecuted and convicted of cruelty.
    •    The 1st time a federally funded research grant had ever been terminated because of cruelty.
    •    The 1st time an entire laboratory had been forced to shut down because of cruelty.
    •    The 1st time the rights of laboratory animals were ever argued before the U.S. Supreme Court.

The undercover work, damning photographs and evidence, arrests, confiscation, and multiple criminal and civil trials shook the mammoth multi-billion dollar international biomedical and pharmaceutical industries as never before.