A comparative analysis of the use of psychoactive plants in northern Chile, specifically San Pedro deAtacama and the Middle Loa and its tributaries ( Caspana, Toconce, Lasana, Chiu-Chiu), with NW Argentina, as a proba ble diagnostic tool of shifting patterns of interaction. This work presents a review of the evidence for the ingestion of psychoactive plants in northern Chile and NW Argentina with particular emphasis on chronology, iconography and modes of ingestion. The iconographie and stylistic differences, as well as the divergence in chronology, suggest shifts in patterns of interaction between northern Chile and NW Argentina. RESUMEN Un an?lisis comparativo del uso de plantas s ?coactivas en el norte de Chile, especialmente San Pedro deAtacama y Loa medio y sus tributarios (Caspana, Toconce, Lasana, Chiu-Chiu) en relaci?n al noroeste argentino, se presenta como una posible herramienta de dign?stico de cambios de los patrones de interacci?n entre ambas regiones. Este trabajo es una revisi?n de las evidencias de ingesti?n de plantas sicoactivas en el norte de Chile y noroeste argentino con ?nfasis en la cronolog?a, iconograf?a y modos de ingesti?n. Las diferencias iconogr?ficas y estil?sticas, as? como la divergencia cronol?gica, sugieren cambios en los patrones de interacci?n entre el norte de Chile y el noroeste argentino. Palabras claves: Vilca, inhalaci?n, plantas sicoactivas, rap?. This work presents a comparative analysis of the evidence provided by the use of psychoactive plants in San Pedro de Atacama, the Middle Loa and its tributaries, and NW Argentina (Figure 1), as a probable diagnostic tool of shifting patterns of interaction. The criteria used include modes of ingestion, iconographie variables, and chronology. The modes of ingestion easily identified in an archaeological context are snuffing and smoking. The paraphernalia employed in these activities is specific to the task. However, the probable use of liquid preparations is not easily distinguishable, since its paraphernalia cannot be separated with any certainty from the wide array of vessels present in these areas. Chemical analysis of archaeological snuffs and of smoking preparations in northern Chile and NW Argentina have detected the presence of several psychoactive alkaloids, most notably bufotenine (5-hydroxy-dimethyltryptamine). The finding of bufotenine in the Atacama snuff suggests that the plant source of this material was a species of the genus Anadenanthera (formerly Piptadenia Benth.; see Reis Altschul 1964; 1972). This is the only genus implicated in the snuffing complex that contains bufotenine (Torres et al. 1991). The species of Anadenanthera present in the South Central Andes is A. colubrina var. Cebil (Figure 2). Small pouches containing Anadenanthera seeds have been found in several burials at the site of Solcor 3, San Pedro de Atacama. Anadenanthera seeds in association with smoking pipes have been found at the preceramic site of Inca Cueva in the Puna de Jujuy (Fern?ndez Distel 1980), and at the late period site of Cusi-Cusi (after 950 A.D.?;