The effect of atmospheric NH3 deposition on freezing tolerance of needles of Scots pine was investigated. Needles from 3 month old seedlings and from 20-30 year old field-grown Scots pine (Pinus sylvestris L.) trees were studied. Seedlings were exposed to NH3 during the first stage of hardening in the laboratory. Field-grown trees were growing in a "clean" or an NH3 polluted area. In the 3 month old seedlings, exposure to 1000 ml 1** (-1) NH3 for 4 weeks resulted in a 40 increase of total nitrogen content. Freezing tolerance of the needles was significantly enhanced in the NH3 exposed seedlings. In the field-grown trees, current and one year old needles were studied. Freezing tolerance of the current year needles did not differ significantly between the two forest stands. However, one year old needles from the NH3 polluted stand were significantly less frost hardened from October until February. Total nitrogen content was higher in both current and one year old needles from the NH3 polluted stand. At the same time, potassium content of these needles was lower compared to the needles from the clean stand. The content of calcium, magnesium and phosphorus did not differ significantly. It was concluded that a decrease of freezing tolerance upon atmospheric NH3 was not directly related to an increased nitrogen content, but might probably be due to the nutrient imbalance of the needles.