Air pollution is one of the most serious environmental threats facing our national parks. In its role as a protector of natural ecosystems, the National Park Service has a responsibility to document the extent and severity of such external threats. Sequoia and Kings Canyon National Parks, in the southern Sierra Nevada of California, are the site of a large, interdisciplinary effort of monitor and evaluate potential effects of ozone, particulates, acidic rain and snow and dry deposition. Summer drought and poorly buffered soils and waters predispose the area to impacts from atmospheric pollutants. Data collected is of value in assessing the threat as well as the extent of actual damage to park ecosystems. Such information will be of use to regulatory decision makers, as well as provide valuable baseline data on park ecosystems. The integrated program includes federal, state and private funding and investigators. Findings to date include the occurrence of acidic deposition, summer ozone concentrations in excess of federal and state standards, evidence of ozone symptoms on foliage, obstructed visibility and documentation of poorly buffered terrestrial and aquatic ecosystems that are sensitive to acidic deposition. The scope and complexity of the overall program provides a model of the role national parks can play in understanding the impacts of man's activities on natural ecosystems.