The present experiment was designed to assess the developmental period(s) during which exposure to methylmercury chloride (MMC) would result in permanent learning deficits in rats. In addition, the mercury (Hg) content of the brain at these different stages was measured. Offspring (30 days of age) of mothers exposed during gestation and offspring exposed directly to MMC for nine days after weaning exhibited the greatest learning deficits on a water escape T maze. These deficits persisted through a retest session 21 days later. Biochemical analysis of brain Hg content indicated that Hg need not be present for these learning deficits to occur.