Biomass burning is influencing the atmospheric chemistry by emitting large amounts of reactive species such as hydrocarbons, organic acids and nitrogen compounds [Andreae et al., 1988]. Polar ice cores provide a unique record of precipitation whose chemistry reflects the atmospheric composition at the time of deposition. The analysis of such ice samples therefore allows an estimate to be made of the concentration of atmospheric impurities in the past. During the first season of the deep drill operation (GRIP) at Summit, Central Greenland (72° 34' N, 37° 38'W) continuous ammonium (NH4+) measurements were performed between 100 and 600 m depth covering the time period from 330 to 2500 years B.P. The NH4+ concentrations show seasonal variations between 1–20 ng.g−1 with some sporadic high values up to 600 ng.g−1 in narrow layers. The chemical fingerprint points to biomass burning causing the high ammonium peaks.