Organochlorine pesticides have been used in Mexico in malaria control programmes against ectoparasites and as seed dresser. Owing to their chemical stability, they tend to accumulate in the lipid part of the organisms. The stored pesticides are excreted with the endogenous fat during milk production. The aim was to monitor the organochlorine pesticide levels in butter manufactured in Mexico. From the pesticides, only HCB, beta-HCH, pp'-DDT, op'-DDT and pp'-DDE with major frequency and levels were detected. The HCB mean level was low at 0.008 mg kg(-1) on a fat basis. From the HCH isomer, only the beta-HCH at 0.065 mg kg(-1) on a fat basis was determined, remaining as the main contaminant of the monitored butters. Among DDTs, pp'-DDE was the major constituent (0.043 mg kg(-1) on a fat basis) followed by pp'-DDT (0.036 mg kg(-1)) and op'-DDT (0.009 mg kg(-1)). Comparing the previous study (1994) and this one (2001), all organochlorine pesticides had a descendent tendency; beta-HCH decreased from 0.095 to 0.065 mg kg(-1) on a fat basis, whereas the total DDT decreased from 0.056 to 0.047 mg kg(-1), pp'-DDT from 0.050 to 0.036 mg kg(-1), op'-DDT from 0.018 to 0.009 mg kg(-1), while pp'-DDE increased from 0.032 to 0.043 mg kg(-1). The decreased DDT levels in Mexican butters is caused by the substitution of organochlorine insecticides with pyrethroids used by the Mexican Ministry of Health since 1999 in sanitary programmes.