Two Mussel Watch programmes have collected and analysed Cd, Cu, Pb, Ni, Ag, and Zn concentrations in bivalve molluscs around the coasts of the United States annually for two periods of 3 yr, one in the late 1970s, the other in the late 1980s. Fifty sites were common to both programmes. The two 3 yr data sets have been compared by the use of two nonparametric techniques to identify changes over the 10 yr separating the programmes. By the Sign test, mean concentrations of Pb were shown to have decreased on a national scale. When the results for each metal were compared station-by-station using the Mann-Whitney U-test, substantially more sites were found with decreases in Cd and Pb than with increases. For Cu the converse held, substantially more sites with increases being noted. On a national scale, Ag, Ni and Zn showed little evidence of change between the two data sets.