Paleolimnological techniques were used to establish baseline environmental conditions as guidelines for lake management and to evaluate water quality changes over the last 300 years in the north basin of Upper Saranac Lake (Adirondack Park, New York). Analysis of diatoms, bulk sedimentary characteristics and fossil pigments in a 30-cm core showed that the lake was once oligo- to mesotrophic but has become increasingly productive since the turn of the century, especially since the early 1950's. Relative abundances of the diatomsFragilaria crotonensis, Tabellaria flocculosa, andAsterionella formosa– common indicators of nutrient enrichment – increased in the most recent sediments, as did loss-on-ignition, cyanobacterial pigments, and inferred total phosphorus, pH, and chloride. Comparison of core profiles with historical data suggests that the productivity increased because of human activities, in eluding shoreline development and the discharge of fish hatchery effluent into a major tributary of the north basin. Short-term variations in diatom-inferred phosphorus since 1950 may be partly attributable to weather conditions.