Vasopressin regulated in vitro the activity of Na+, K+-ATPase in synaptic membranes from rat brain cortex and brain stem. Low concentrations of the peptide activated the enzyme, high concentrations--inhibited. The effect was most distinct in brain stem. Age-dependent effects of vasopressin on the Na+, K+-ATPase activity were observed. Basal activity of the enzyme was not altered in synaptic membranes with ageing.