Like halothane, cyclopropane reduces stimulation-induced release of norepinephrine, but not release of dopamine-β-hydroxylase, from the isolated guinea-pig vas deferens. The dissociation between transmitter release and enzyme release in the presence of cyclopropane may be the result of either an increase in the affinity of norepinephrine for binding sites on the vesicular membrane produced by cyclopropane, or a direct effect of cyclopropane on a mechanism of release of norepinephrine that could be controlled independently of release of dopamine-β-hydroxylase.