Sources of methyl mercury (MeHg) in the Great Lakes Basin are not completely known, but tributaries and atmospheric sources are thought to be two of the main contributors.  The specific mechanisms of MeHg formation in the atmosphere is not presently understood.  Nevertheless, there are detectable MeHg levels (> 0.03 ng/L) in many rain and snow samples.  As part of an ongoing Hg monitoring program in the Lake Superior Basin, precipitation samples have been collected on an event basis over the last two years by the University of Michigan and analyzed for MeHg by the University of Wisconsin.  Specific sites sampled include Brule, WI; Isle Royale National Park; Eagle Harbor, MI and Tahquamenon State Park, MI.  In general, the highest MeHg levels in rain have been seen in samples collected from Isle Royale while the lowest levels have been observed in precipitation at Tahquamenon State Park, following a pattern of decreasing concentrations west to east across the basin.