Soil and alfalfa samples (Medicago sativa L.) were collected in five locations of three valleys of the Arica Province, which are irrigated by their respective rivers, with different arsenical concentrations. The objective was to describe the levels and availability of arsenic (As) in the soil samples, as well as the soil-plant relation, in terms of the absorption and magnification of As by alfalfa. Results indicated a different level of soluble As in the soil samples and total As in the alfalfa samples. 80th variables showed the same gradient, with a significant regression (R2 = 0.856) between them, indicating that the As absorption by the alfalfa plants is mainly dependent on the arsenical Intensity of the soil solution. The grade of arsenical mangnification shows a negative regression (R2 = 0.781) on the arsenical I ntensity of the soil solution. Alfalfa crops from the Camarones valley, with the highest arsenic level, should not be toxic when used as green forage, but when dried to 40% humidity or less, As content gets above the safety toxic leve!.