Previous work has reported behavioral arousal in the rat to be inversely related to cortical GABA production. Therefore the effects of an increase in brain GABA levels, induced by amino-oxyacetic acid, on measures of behavioural arousal such as rearing and ambulation were examined. The increase in GABA was immediately associated with decreased rearing, however the behaviour was later indistinguishable from control values while the levels of brain GABA remained raised. It was suggested that the return to normal behaviour that occurred while brain GABA levels were increased, reflected an interaction between excitatory and inhibitory systems, and that a compensation had occurred to return the balance to normal. The pre-treatment with alpha-methyl-p-tyrosine, known to deplete brain noradrenaline and dopamine, prevented the characteristic return of normal behaviour that followed the injection of amino-oxyacetic acid. This data is consistent with GABA and noradrenaline or dopamine-mediated systems interacting in the control of behavioural arousal. The depletion of serotonin with p-chloro-phenylalanine did not prevent the characteristic recovery of behaviour that followed the injection of amino-oxyacetic acid.