The bright yellow one is called Pentoject (active ingredient pentobarbitone sodium), it's what we use.
With dogs, it's usually IV. With cats it's also usually IV, although sometimes cats have to be sedated first. If they're very flat, or sedated, we sometimes inject straight into heart as it's quicker.
With small furries it's generally 'intra-hamster' (or whatever the animal is)! Although you can sometimes get a good vein in rabbit ears.
With all these animals, if they're very ill and their blood-pressure is rubbish, it can be extremely hard to get a vein to inject into.
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With small mammals we do gas them down first in an anaesthetic chamber.