The German government has denounced experiments funded by German carmakers in which humans and monkeys reportedly inhaled diesel exhaust fumes.
German media say the health impact research was done by EUGT, a body funded by Volkswagen, Daimler and BMW.
Such tests could not be justified, the government said, demanding details. A minister called them "abominable".
Daimler also condemned them. VW is embroiled in a scandal over software that gave false diesel exhaust data.
In 2015 VW admitted having fitted "cheat" devices in the US that made their engines appear less polluting than they actually were.
EUGT was dissolved by the carmakers last year. The initials stand for European Research Group on Environment and Health in the Transport Sector.
"These tests on monkeys or even humans cannot be justified ethically in any way," said Chancellor Angela Merkel's spokesman, Steffen Seibert.
Environment Minister Barbara Hendricks called the experiments "abominable" and expressed shock that scientists had agreed to conduct them.
Social Democrat politician Stephan Weil - a VW supervisory board member - called them "absurd and abhorrent". "Lobbying can be no excuse whatsoever for such testing," he said.
