1. Origin of methylmercury exposure: Marine food is the main source of methylmercury, concentration usually required to be below 0.5 µg/g. White seabream from the Azores contained mercury concentrations up to 24.6 µg/g in the 20-30 cm size range. 2. Population groups at risk: The hair-mercury concentration was used as indicator of current exposure to methylmercury; it should be kept below 10 µg/g in pregnant women. No excess exposure was found in the Azores, but pregnant women from the fishing community of Câmara de Lobos, Madeira, showed an average of 11.5 µg/g. 3. Methylmercury retention in children: Detailed health examinations of children (N = 1022) were carried out in the Faroe Islands, where maternal hair-mercury concentrations at parturition averaged 4.5 µg/g. Highly-exposed Inuit children (N = 43) from Thule, Greenland, and a group of children from Câmara de Lobos, Madeira, (N = 162) were also examined. 4. Mercury-associated effects on the nervous system: In addition to standard clinical and neuropsychological tests, we added computer-assisted tests from the Neurobehavioral Evaluation System (NES) to assess motor speed, vigilance and motor coordination. Also, evoked potentials were added to represent objective measures of physiological processes in the brain. Delays on evoked potentials occurred in Madeira at high exposure levels, but no such delay was found in the Faroe Islands. On the computer- assisted tests, especially the reaction time measure on a continuous performance task showed mercury-related delays. Thus, a doubling in mercury exposure was associated with an increase in reaction time of 8 msec. This result corresponds to a delayed brain development of 1-2 months. These tests therefore document the usefulness of modern testing techniques and indicate adverse effects at exposures hitherto thought to be safe.