To monitor the exposure of Canadian populations, Health Canada has carried out six major surveys of breast milk contaminants, beginning in 1967. In these national surveys, breast milk was collected from nursing mothers at various times in their nursing period, and the concentrations of chlorinated hydrocarbons were measured. There was a downward trend from 1967 to 1992, in the concentrations of organochlorine (OC) pesticides and polychorinated biphenyl hydrocarbons (PCBs). This regression was observed for samples across Canada, except for a sharp isolated increase in 1982 for the Quebec Province. As the disappearance curves of the chemicals surveyed follow the same pattern for both Ontario and Canada, a similar decline could be expected of OC concentrations in human breast milk in the Great Lakes Basin.