1 In rabbit isolated thoracic aortae, effects of magnesium ions on the contraction and (3)H-efflux in response to electrical transmural stimulation and on the contractile responses induced by noradrenaline and KCl were investigated.2 Addition of magnesium (1.2, 3.6, 12.0 and 24.0 mM) to the bathing solution inhibited the electrically induced contractions in a dose-related manner; the inhibition was complete with a concentration of 24.0 mM.3 The increase in (3)H-efflux induced by electrical transmural stimulation was inhibited by the addition of magnesium to the superfusing fluid, but a complete block was not obtained even in high concentrations of magnesium.4 Magnesium ions (1.2, 3.6, 12.0 and 24.0 mM) inhibited the contractile responses induced by low concentrations of noradrenaline (2 x 10(-8)M) and KCl (2 x 10(-2)M). However, the responses induced by higher concentrations of noradrenaline (5 x 10(-7) and 10(-5)M) and KCl (3 and 4 x 10(-2)M) were enhanced by low concentrations of magnesium.5 Magnesium ions affect both presynaptic and effector sites in rabbit thoracic aortae but in a different manner; magnesium manifests only an inhibitory effect on noradrenaline release from the adrenergic nerves, but dual effects on reactivity of vascular smooth muscle, depending on concentrations of magnesium and stimulants; it is suggested that the change in vascular reactivity is more important than the reduction in transmitter release when magnesium inhibits the response to nerve stimulation.