The effect of acute swim stress on the functional activity of the gamma-aminobutyric acid (GABA) receptor/chloride ion channel was studied using an assay to measure 36chloride (36Cl-) uptake into rat brain synaptoneurosomes. Muscimol-stimulated 36Cl- uptake in cerebral cortex, hippocampus, but not cerebellum, was enhanced (increased potency and efficacy) in rats subjected to 10 min of swimming, compared to non-stressed, control rats. The effect of swim stress on the activity of the GABA receptor/Cl ion channel was prevented by adrenalectomy.