The German car industry has faced fresh criticism following reports that diesel exhaust tests were carried out on both monkeys and humans.
The chairman of Volkswagen, Hans Dieter Poetsch, said that "in the name of the whole board I emphatically disavow such practices", while the German government also condemned the experiments.
Revelations of the tests add a twist to the German car industry's attempt to move past Volkswagen's scandal over cheating on diesel tests and the resulting questions over diesel technology across the industry.
Mr Poetsch said the tests must be "investigated completely and without reservation", according to the dpa agency.
A report by the New York Times found that a research group funded by major German car companies commissioned experiments in which one group of monkeys was exposed to diesel exhaust from a late-model Volkswagen, while another group was exposed to fumes from an older Ford pick-up.
The experiments were carried out in 2014 before Volkswagen was caught using software that let vehicles cheat on emissions tests. They were intended to show modern diesel technology had solved the problem of excess emissions, but according to the New York Times report the Volkswagen car in the tests was equipped with illegal software that turned emissions controls on while the car was on test stands, and off during regular driving.
Volkswagen admitted using the software in 2015. The Volkswagen scandal led to public scrutiny of diesel emissions as regulators discovered that other companies' vehicles also had higher emissions on the road than during testing, though not necessarily through illegal rigging. The industry has had to fend off calls for diesel bans in German cities with high pollution levels.
The New York Times report was followed by one in Monday's edition of the Stuttgarter Zeitung daily, which said that the now-closed research group also commissioned tests in which humans were exposed to nitrogen dioxide, which belongs to a class of pollutant known as nitrogen oxides. The group reportedly said the tests showed no effect on the subjects.
The human study, carried out by Aachen University, involved exposing 25 subjects, mostly students, to low levels of nitrogen oxides. The individuals gave informed written consent for the study, which was approved by the ethics committee of the university's medical faculty, according to the study.
Daimler AG said it was "appalled by the nature and extent of the studies" and said that, though it did not have any influence on the studies' design, "we have launched a comprehensive investigation into the matter".
BMW said that it "did not participate in the mentioned study" on animals "and distances itself from this study". It said it was investigating the work and background of the research group. The Times report said the group, known by German initials EUGT, got all its funding from the three car makers.
Chancellor Angela Merkel's spokesman said "the disgust many people are feeling is absolutely understandable".