Today's News & Observer details efforts to get NC pet shelters to switch from gas chambers to lethal injections for killing animals:
[D]ozens of North Carolina shelters  still use gas chambers to kill unwanted dogs and cats. The practice is falling out of favor nationally, and some animal activists are lobbying the state to stop it.
As I understand it, though, the main objection to the gas chamber is that it's the gas chamber. One activist says "it's a pet holocaust." Both methods are painless and effective, though injection is slightly quicker—20 seconds of consciousness after injections, 30-60 after the gas is turned on.