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Ion input via rainfall alone and after interception by the forest canopy, constitutes an important path of biochemical cycling, although few studies have provided information on the subject so far. The objective of this work is to quantify ion inputs, via rainfall, in the southwestern region of Rio Grande do Sul state, Brazil. Ten rain gauges were mounted in a field area. The quantification of stored water volume, along with sample collection for determination of nitrate, nitrite, ammonium, phosphorus, sulfur, chlorine, calcium, magnesium, potassium and sodium contents, was done fortnightly from September 2006 to August 2008. Local annual average precipitation in the relevant period was 1,588.3 mm. The concentration of chemical elements in rainwater was found to vary throughout, being inversely correlated with the increase in rainfall, while potassium, ammonium, phosphate, sulfate, chloride and sodium were found to have a significant mutual correlation (p <0.01). Based on the annual amount of nutrient input via rainwater, it can be inferred that rainfall is an important source of chemical element input into the forest system.