The “San Pedro Macho” cacti (Echinopsis peruviana or also known as Trichocereus peruviana) owes its name and importance, in part, to its taxonomical and alkaloid composition similarity to the well-known “San Pedro Hembra” cacti (Echinopsis pachanoi) or commonly referred as “San Pedro”, which is the most important species among the plants used in Traditional Medicine (TM) in Northern Perú. This phytochemical report of an Echinopsis peruviana species cultivated in Perú involves the qualitative analysis of its secondary metabolites, the quantification of the total alkaloid content and the isolation of the alkaloid mescaline, as its sulphate dihydrate salt, of the chlorenchyma in the green outer cortex of the stems of E. peruviana. ESI-MS; 1H, 13C NMR; IR; UV; elemental analysis and TLC techniques were used to characterize this compound.