Two series of experiments have been carried out to study the relative contributions of both dry deposition wash off and crown leaching, including H⁺ -cation exchange, to the change in chemistry of rain passing over the leaves of Sitka spruce (Picea sitchensis (Bong.) Carr.). In the first experiments detached needles and small trees were washed under several conditions with artificial acid rain water, following various pretreatments. Increases in Na and Mg in the canopy drip were found to be due, predominantly, to washed off dry deposition whereas about 50% of the K and Ca increases on washing with solutions of pH less than 4 were considered to be due to leaching of neutral salts and H⁺ -cation exchange. In a second series of experiments the unidirectional membrane fluxes of ions across the needle surface were studied by compartmental analysis, using ³⁶Cl, ⁴⁵Ca and ⁴²K. There appears to be a net loss of all these elements when artificial rain water solution of pH 4.3 was used for elution. Neutral salt leaching over 24 h may result in as much as 5% of total K present in needle being lost, making it the most readily leached cation. Ca is not readily available for exchange as it is bound within the vacuole and cell wall. It was determined that about 15% of total Ca is available for exchange, of which 1.5-3% is considered to be leached as neutral salts. The elution of ⁴⁵Ca from needles attached to twigs suggested that leached Ca may be readily replaced by movement from the translocation stream to the cytoplasm. Although Cl in canopy drip is generally considered to be due predominantly to washed off dry deposition, about 12.5% of the total Cl content of needles was estimated to be leached over 24 h.