A group of 10 macaque monkeys were forced to inhale diesel fumes while watching cartoons in airtight chambers, according to the New York Times.
The 2014 tests were part of a bid by German automakers - including Volkswagen, Daimler and BMW - to prove that vehicles with new diesel technology have cleaner emissions than old ones.
But according to reports, the car emitting the fumes - a Volkswagen Beetle - had been rigged to produce deceptively low pollution levels.
Daimler and BMW have denied any knowledge of the tests being rigged.
While the emissions rigging scandal became public some time ago - with Volkswagon forced to pay $26 billion in fines after pleading guilty to federal fraud and conspiracy charges in the United States - news of the monkey experiment has only now reached the public domain.
According to details, cars were set up on rollers, and exhaust was sucked from the tailpipe, diluted, and fed into the chambers the monkeys were in.
The monkeys were reportedly shown cartoons in order to keep them calm during the four-hour experiment, with one of the scientists saying: "They like to watch cartoons."
Animal rights charity PETA has spoken out before about experiments on primates including monkeys.
According to PETA: "While it is well known that nonhuman primates are sensitive, intelligent beings who share many important biological and psychological characteristics with humans, these very attributes, unfortunately, make them prime targets for experimenters, who treat them as if they were disposable pieces of laboratory equipment."