The ultimate end point among the various measures of neurotoxicity is the death of neurons, since they do not regenerate. Experiments that accurately assess this degree of neurotoxicity must utilize a sensitive means of detection and control for the factors that are variables in the expression of neurotoxicity. Neurotoxic substances have the characteristic trait that each selectively destroys specific populations or subpopulations of neurons. The mechanisms underlying this selectivity are generally unknown. Without a priori knowledge of where in the brain to look, it is prudent to employ a histological procedure that imparts an easily detected marker to degenerating neuronal components. For example, the so-called "reduced silver" stains specifically impregnate degenerating neural elements. From studies of known neurotoxins, several factors have been shown to influence neurotoxicity: 1) species and strains; 2) age; 3) sex; 4) dose rate (acute vs. chronic); 5) survival time; and 6) sampling interval in the brain. Failure to include these factors in a screening paradigm could yield false-negative results and, thereby, invalidate extrapolations made to man.