Both sulfur and nitrogen pollutants may contribute to acidification of the environment. Meausrements indicate that nitric acid is an important contributor to the acidity of snow-melt but that sulfuric acid is the major contributor to rain acidity. In summer nitrate may be utilized by biological processes which effectively neutralize one hydrogen ion per nitrate utilized. In the winter these processes may not be rapid enough to utilize the nitric acid in snow-melt before it reaches lakes and streams. Therefore, NOX may be of concern during the winter, especially in regard to acidification of lakes and streams and spring fish kills. A budgeting exercise for SO2 and NOX leads to an estimate that passenger cars contribute 5% of annual wet deposition of acidity in the U.S., 8.5% of total (wet plus dry) acid deposition, and 11% of the acidity in snow-melt, as measured at several sites in the rural, northeast U.S.