The analysis of visual toxicity is complicated by the heterogeneity of visual capacities in different regions of the visual field. Since various toxicants may impair different functions allied to localized portions of the visual field, it is important to explore the relationship of field defects to residual visual abilities. We have begun this exploration by studying methylmercury poisoning in macaque monkeys. Extended exposure to this toxicant produces a marked concentric constriction of visual fields, a result similar to that found in human victims. In addition, visual sensitivity is greatly reduced on those tests in which the periphery of the visual field is more sensitive than the center. Our findings suggest simple but reliable clinical tests for screening suspected victims of substances impairing peripheral vision.