An array of sediment cores was analyzed to determine historical trends in mercury (Hg) accumulation in Lake Pepin, a natural lake on the Upper Mississippi River. Whole-basin Hg accumulation rates increased from 3 kg/yr before European settlement (ca. 1830) to a maximum of 357 kg/yr in the 1960s. The recent Hg accumulation rate (110 kg/yr, 1990-1996) is experimentally indistinguishable from measured Hg loadings in the river entering the lake, indicating that accumulation rates in Lake Pepin correspond quantitatively to river loadings. The modern accumulation rate represents a decline of almost 70% from the peak value, reflecting large decreases in Hg inputs to the Mississippi River from industrial and municipal point sources in the metropolitan Minneapolis/St.Paul area upstream. A total of 18.1 t of Hg has been deposited in Lake Pepin since 1800; half of that (9.0 t) was deposited between 1940 and 1970, when regional growth accelerated rapidly but pollution control mechanisms were inadequate. Point sources accounted for approximately 60% of the Hg accumulating in Lake Pepin in the 1960s, but these inputs have been virtually eliminated since then.