A method and a system (2) for gas stunning of animals for slaughter is described, where the animals arrive at the slaughterhouse in transport crates (6), where gas stunning of the animals is effected while they are still in transport crates (6), and where the transport crates with the animals, a number of conveyors (12, 14, 18, 20), are conveyed successively through a stunning chamber (8), where the action of the gas for stunning of the animals is adjusted by shortening or prolonging the conveying time and / or the conveying route of the said transport crates (6) through the stunning chamber (8). It is possible to optimize stunning while at the same time considering all the parameters. If the stunning condition of the animals is not optimum, it is easy to prolong or shorten the conveying time and / or the conveying route through the stunning chamber.