Neurotoxic effects of styrene on certain biochemical and behavioural indices were studied in young rats kept deficient in protein during the weaning period. Young rats fed a low-protein diet showed a significant decrease in the level of dopamine and an increase in frontocortical [3Hlserotonin binding in comparison to the group of rats fed a normal protein diet. These rats also showed a significant increase in foot shock-induced aggressive behaviour, while no changes in amphetamine-induced locomotor activity, levels of norepinephrine and serotonin and binding of [3H]spiperone to striatal membrane were observed. On exposure to styrene, rats fed a normal protein diet showed a decrease in dopamine level and an increase in foot shock-induced aggressive behaviour only, with no significant change in other parameters, in comparison to the respective controls. It was, however, interesting to note that when rats fed a low-protein diet were exposed to styrene they showed a significant decrease in the levels of norepinephrine, dopamine and serotonin and an increase in the binding of [3H]spiperone and ['HIS-HT to striatal and frontocortical membranes, respectively. A significant increase in foot shock-induced aggressive behaviour and amphetamine-induced locomotor activity was also observed in this group of animals in comparison to those fed a low-protein diet. The biochemical and behavioural data indicate that protein deficiency makes young animals more vulnerable and it is an important predisposing factor in the neurobehavioural toxicity of styrene.