1. Isolated rabbit arterial segments were perfused with buffer containing sodium (155 mmol/l) and vasoconstrictor responses to noradrenaline, histamine and electrical stimulation were measured. When the concentration of sodium in the perfusing medium was raised by 8 mmol/l, the vasoconstrictor responses to all stimuli were significantly increased, though response to electrical stimulation was less enhanced than the responses to the other stimuli.
2. The constrictor response of the isolated perfused rabbit ear to angiotensin was also enhanced by perfusion with buffer of a similarly enriched sodium content.
3. Ileal responses to electrical stimulation and atrial responses to electrical stimuli and noradrenaline were also examined after exposure to similar changes of environmental sodium. No change in responsiveness was demonstrated.
4. Isotope-exchange studies with 22Na showed that this small increase in sodium concentration of the environmental fluid caused an increased sodium ion content of 20% in arterial wall, 5% in ileum and 11% in atrium.
5. It is suggested that a relationship exists between increased exchangeable sodium ion in arterial wall and arterial reactivity.