The company said on Monday a small internal group had mistakenly pushed for the tests to be carried out and that they did not reflect VW's ethos.
Europe's largest automaker has come under fresh scrutiny after the New York Times said last week that it and German peers BMW and Daimler funded an organisation called European Research Group on Environment and Health in the Transport Sector (EUGT) to commission the tests. Doctors at an Aachen-based university hospital then examined 25 people after they inhaled varying concentrations of the gas over several hours.
The German government has condemned some of that country's biggest carmakers, over media reports that say humans and monkeys were used in experiments to test the dangers of exhaust fumes. Things changed in June 2012 when the World Health Organization labeled the exhaust emissions generated by diesel engines as being "carcinogenic to humans", thus forcing those involved in organizing these tests to resort to something else. Barbara Hendricks, the German environment minister, described the experiments as "abominable".
An environmental campaign group is calling on Volkswagen to end diesel sales, following allegations that the carmaker tested its emissions on monkeys.
Daimler, manufacturer of Mercedes-Benz cars, said on Sunday "we are appalled by the extent of the [EUGT] studies and their implementation".
In what seems like a John Henry versus the steam shovel-style competition to dig diesel's grave, Volkswagen has admitted to funding (and subsequently cheating on) animal testing to prove the relative safety of diesel exhaust fumes, according to findings by the New York Times.
VW is no stranger to controversy following the 2015 dieselgate scandal where the carmaker admitted fitting software created to cheat emissions testing to 11 million vehicles. The trials were "abhorrent and repulsive", Stephan Weil, prime minister of Lower Saxony and a member of VW's supervisory board, said.
Alleged fresh evidence that leading German auto makers commissioned unethical experiments to demonstrate the limited health risk posed by modern diesel engines prompted angry responses from politicians and animal rights activists on Monday. That was despite VW knowing the Beetle used in the experiments contained a "cheat" device.
Reacting to the NYT report about the monkey experiments, VW tweeted on Saturday that it "explicitly distances itself clearly from all forms of animal abuse".
In a second round of tests, the animals were forced to breathe in the fumes of a Ford F-250 used for the purposes of comparison, because the vehicle was an older model with apparently less sophisticated filter technology.
While it appears the university secured ethics approval to conduct the research - in addition to written consent from all those who took part - it's just another black mark against the carmakers implicated in the diesel emissions scandal, especially with what we know about how bad nitrogen dioxide is for us.
BMW said it "does not carry out any animal experiments", adding that: "The BMW Group in no way influenced the design or methodology of studies carried out on behalf of the EUGT".
The German establishment as a whole widely condemns the tests.
Responding to widespread reports that humans were harmed in automotive industry experiments, however, Professor Thomas Kraus, who led the controversial 2013 study at the University of Aachen, emphasized that it was not related to diesel engines.