Passerine species have been increasingly used as monitors of metal pollution, especially by making use of non-destructive indicators of bird exposure, such as collecting feathers, faeces or blood. During this study, mercury concentrations were determined in feathers, faeces and blood of nestling great tits (Parus major) in industrial (a paper mill) and rural sites on the west coast of Portugal. The aim of this study was to compare the level of mercury in both areas over the study period, as indicated by nestlings' mercury levels, while assessing possible contamination effects on the breeding performance and health status of great tits. Over the years, feathers showed a significant annual decrease in mercury contamination in the study area. Blood analyses also revealed a significant annual decrease in mercury concentrations, but no significant differences were detected between areas. Faeces data showed no significant difference between years or areas. We found no direct influence of mercury levels on nestling health status or great tit breeding performance.