Noradrenaline (NA) uptake was measured in the anococcygeus by two methods: quantitative fluorescence microscopy and radioactive tracer techniques. A pharmacological analysis of the contribution of adrenergic nerve, smooth muscle, and nonspecific binding to collagen and basement membrane was attempted by studying the effects of desmethylimipramine and phenoxybenzamine on uptake. The importance of metabolic degradation in the process of NA accumulation was estimated by examining the effect of the combined use of iproniazid to inhibit monoamine oxidase and of β-thujaplicin to inhibit catechol O-methyltransferase.The results suggest that NA accumulation in this tissue at concentrations below 2 × 10−6 g/ml was almost entirely due to uptake into adrenergic nerves and to nonspecific binding. At higher concentrations of NA, above 2 × 10−6 g/ml, uptake into the smooth muscle cells began to contribute increasingly to the total accumulation. Inhibition of the metabolic enzymes increased the fluorescence of smooth muscle incubated with high concentrations of NA, confirming that only at those high concentrations was there any intracellular accumulation of NA and suggesting that the NA taken up by these smooth muscle cells is normally almost completely metabolized. Incubation with the inhibitors of catecholamine metabolism, and also with phenoxybenzamine, reduced both the fluorescence in the adrenergic nerve terminals and the accumulation of tritiated compounds by the whole tissue. Possible explanations for this observation are discussed.In the radioactive experiments 14C-sorbitol was used to measure the extracellular space (ECS). The ECS appeared to be a two-compartment system, a large, fast-filling and a small, slow-filling compartment. Electron micrographs suggest a possible basis for these two compartments in the large spaces between muscle bundles and the very narrow clefts between the cells in individual bundles. Quantitative estimates of the magnitude of these two spaces from electron micrographs are not incompatible with this.