During the reproductive cycle significant fluctuations of concentrations of organochlorine pollutants were detected in blood and uropygial oil from Adelie penguins (Pygoscelis adeliae) and southern fulmars (Fulmarus glacialoides) from Hop Island, Antarctica. In the Adelie penguin, concentrations of polychlorinated biphenyls (PCBs) and dieldrin were significantly higher at the moment of egg laying, while hexachlorobenzene (HCB) and 2,2-bis(p-chlorophenyl)-1,1-dichloroethylene (p,p'-DDE) concentrations peaked at the creche stage. Subcutaneous fat is the main pool for HCB and p,p'-DDE, while PCBs and dieldrin are also stored n the pectoral muscle. It is hypothesized that the observed differences in seasonal fluctuations in organochlorine levels are related to utilization of different storage pools by the Adelie penguins at different moments in the breeding season. In the southern fulmar the fluctuations in concentrations of HCB and p,p'-DDE can be related to changes in body mass. However, concentrations of PCBs and dieldin did not vary significantly during the season. It seems that the two groups of compounds are governed more or less independently in the southern fulmar as well. Age-dependent accumulation of most organochlorines studied reached a steady state before the age at which both species start to breed. This allows monitoring of individual birds to analyze changes in concentrations of organochlorine pollutants in the ecosystem, without having to correct for age differences. Differences in levels of organochlorine pollutants between the Adelie penguin and southern fulmar were probably the result of differences in feeding ecology.