Mescaline is the primary psychoactive alkaloid in a range of psychedelic cacti, including peyote (Lophophora williamsii), San Pedro (Echinopsis pachanoi), and Peruvian torch (Echinopsis peruviana)—all of which are native to the southwestern United States and northern Mexico. It can also be found in trace amounts in other regional plants, such as the Berlandier acacia (Senegalia berlandieri).
In its natural state, mescaline has been used for thousands of years in Native American religious ceremonies. It was also the first psychedelic to enter mainstream Western culture, predating the widespread use of LSD and psilocybin. More recently, the extracted compound has shown promise in the medical and psychotherapeutic treatment of substance abuse and depression among other conditions.
Pure mescaline is usually available as a white or brownish crystalline powder, either loose or packed into capsules. It can also be found as a liquid solution or brew. Compared to many other psychedelics, however, extracted mescaline tends to be rare in most parts of the world.
The fine folks at The Third Wave have put together a wonderfully thorough guide that covers this remarkable naturally-occuring psychoactive alkaloid.