During the late 1980s, the upper trophic-level biota of the Everglades (FL, USA) was recognized as being highly contaminated with mercury (Hg). However, the timing and pattern of that increase is poorly known, and no information is available about mercury contam ination in Everglades wildlife prior to 1974. We measured methylmercury concentrations in feathers of white ibises (n = 33), great egrets (n = 7), anhingas (n = 21), and great blue herons (n = 12) from museum specimens collected from 1910 through 1980 and combined them with more recent feather samples collected from live birds (1985–2000, n = 98, 37, 49, and 7, respectively). We found no evidence of contamination of museum samples with inorganic mercuric preservatives (0.01–0.28% of total Hg in feathers). All species showed relatively low concentrations of mercury through the 1970s (<5 μl/L dry wt for anhingas, ibises, and egrets, <10 μl/L for herons). Samples from all species taken during the 1990s showed a large and significant increase (4–5X) in MeHg concentration. This evidence suggests that most of the increase in Hg deposition during the 20th century in south Florida occurred during the last two to three decades, which is consistent with information about local source deposition. Contamination levels prior to the 1970s appear to have been associated with normal reproduction in these birds, suggesting partial evidence for a threshold of reproductive impairment.