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Approximately 6,000 dead Swainson's hawks (Buteo swainsoni) were discovered in the Pampas of Argentina in 1995–96. Subsequent meetings held between Argentine, US, Canadian, and Swiss personnel led to an agreement whereby chemical companies removed the organophosphate (OP) monocrotophos (MCP) from the area where mortalities had occurred in northern La Pampa province. We monitored hawks in the MCP exclusion zone from 28 November 1996 through 25 January 1997. We sampled 133 hawks for exposure to anti-cholinesterase insecticides, 131 of which showed no apparent signs of adverse effects. Two hawks had inhibited ChE that recovered over time. Feather and footwash samples from captured birds were tested for OP residues. One feather sample was positive for dimethoate; no footwash sample was positive for any of the OPs screened. Cholinesterase (ChE) activities from hawks in Argentina were (mean±SE, n=131) 0.674±0.014 total ChE, 0.236±0.006 acetylcholinesterase (AChE), and 0.438±0.013 butyrylcholinesterase (BChE). Mean plasma AChE from Argentine hawks was significantly depressed below North American reference values. Total ChE and BChE activities were not significantly different. Mortality due to OP poisoning was not found in the MCP exclusion zone in northern La Pampa, though it continued outside of the zone where MCP was used, both legally and illegally. Education campaigns and extension efforts, emphasizing the local MCP restrictions, were effective in controlling Swainson's hawk mortalities in the study area where substantial mortality occurred the previous two years. [ABSTRACT FROM AUTHOR]