The small, but vital, niche of nonhuman primates in neurotoxicology is examined. Several models of sensory and cognitive function have been especially useful with primates. Their sensitivity to low doses is clear. The validity of data from these models is indicated by their high correlation with data from intoxicated and normal humans, by the degree to which they approximate job functions and other vital human performances, and by their ability to document specific changes in behavioral function which correlate well with morphological and biochemical effects. The use of primates for this research is justified by the absence of adequate alternatives using nonprimate species, in vitro tests or computer programs. A series of experiments on the effects of methylmercury is used to illustrate ethical and scientific issues concerning research with primates. Recent trends are illustrated by data with trimethyltin.