Some individuals are more susceptible to chemical exposure than others, but underlying reasons remain largely obscure. We are investigating these inter-individual differences to gain better understanding of the underlying causes of variability in chemical susceptibility and in addition to obtain insights in the molecular mechanism of action of environmentally relevant chemicals. To study this, we are using zebrafish behavioral assays as a method to sort chemically-exposed individuals into susceptible and tolerant categories, and subsequently analyze their molecular profiles. Zebrafish locomotor behavior can serve as an indicator of perturbations in the nervous system and can be assessed conveniently in a standard well-plate for a large number of larvae simultaneously. Moreover, major brain structures and physiological processes are conserved among phyla, allowing not only neuroactive substances (pharmaceuticals, pesticides etc.) to be analyzed, but also basic findings to be translated to other species, such as other teleosts, and to some extent to vertebrates in general. This project aims to elucidate toxicological response pathways at the molecular level, a much needed basis for a proper risk assessment of contaminants in the aquatic environment, and in turn might potentially identify new contaminant specific biomarkers.