Effluxes of 3,4-dihydroxyphenylalanine, 3,4-dihydroxyphenyglycol, and norepinephrine from four superfused canine blood vessels (saphenous and portal veins and mesenteric and pulmonary arteries) were studied under basal conditions and during nerve stimulation. From quantification of the compounds a series of indices of activities at neuroeffector junctions are proposed. These are (a) basal overflow of 3,4-dihydroxyphenylglycol as an index of vesicular-cytoplasmic translocation of norepinephrine, (b) the increase in 3,4-dihydroxyphenyglycol overflow attributable to nerve stimulation as an index of neuronal reuptake of norepinephrine released by stimulation, (c) the sum of the increases in overflows of norepinephrine and 3,4-dihydroxyphenylglycol attributable to nerve stimulation as an index of evoked release of norepinephrine, and (d) the efflux of 3,4-dihydroxyphenylalanine as an index of the activity of tyrosine hydroxylase, the rate-limiting enzyme in the synthesis of norepinephrine. There were clear differences between these indices in the vessels. Correlation coefficients of the indices among vessels indicated that a high tissue norepinephrine level was associated with high biosynthetic capacity and high vesicular-cytoplasmic exchange but not with high release. There was no evidence suggesting feedback inhibition of synthesis by neuroplasmic norepinephrine--whether arising from vesicular-cytoplasmic translocation or from reuptake from the junctional cleft. The major value of these indices will probably be in determining the integrated effects of pharmacologic agents at neuroeffector junctions in different blood vessels.