Mescaline, a psychedelic substance that exists naturally in peyote and certain other species of cacti, is a Schedule I narcotic—as is peyote itself. Its use is mostly illegal in the United States.
An exception was made, at the federal level, for members of the Native American Church allowing their use of peyote for religious purposes. In certain states, such as Arizona, the law is more lax, allowing anyone to use peyote so long as it is “in connection with the bona fide practice of [any] religious belief.”
As mentioned in How to Change Your Mind, the San Pedro cactus is another source of mescaline. Unlike peyote, it is perfectly legal to grow, although preparing and ingesting it is still illegal in most jurisdictions.
The cultivation, possession and sale of mescaline that isn’t derived from peyote has been decriminalized in Seattle. The successful Decriminalize Nature Oakland initiative decriminalized mescaline, psilocybin and other plant-based entheogens in that city.