Motor activity is a behavioral test that has been recommended as a component of testing batteries that evaluate the neurotoxic potential of chemicals. This brief commentary will address the role of this test in such evaluations. It is organized in accordance with the questions that were posed to members of the Work Group on Activity and Observational Data by the conference organizers. For the purposes of this commentary, 'neurotoxicity' is defined as an unwanted effect of an agent on the structure or function of the nervous system. This effect can be direct or indirect, and transient or permanent. in addition, assessment of neurotoxicity occurs in two stages: hazard identification and characterization.