1 The Falck-Hillarp histochemical technique was used to locate extraneuronal sites of accumulation of isoprenaline in trachea and atria from guinea-pig and cat. With a tissue exposure time to formaldehyde gas of 3 h, isoprenaline was located as green fluorescence.2 Quantitative microphotometry was used to measure fluorescence intensity within cells in the trachealis smooth muscle and the atrial myocardium of both species.3 After incubation of tissues in 50 muM isoprenaline, specific fluorescence was seen in trachealis smooth muscle of both species and in the atrial myocardium of cat but not guinea-pig. In both species, fluorescence was also seen in the chondroblasts of the tracheal cartilage and in blood vessels in all tissues.4 In trachealis smooth muscle of both species and in cat atrial myocardium, fluorescence brightness, resulting from incubation of tissues in 50 muM isoprenaline was significantly increased by 200 muM beta-thujaplicin, an inhibitor of catechol-O-methyl transferase (COMT). In the presence of beta-thujaplicin, fluorescence was not visible in guinea-pig atrial myocardium with 50 muM isoprenaline, although fluorescence brightness measured in myocardial cells was now greater than that in corresponding controls.5 The fluorescence intensity seen in cat and guinea-pig trachealis smooth muscle cells and in cat atrial myocardial cells after incubation in 50 muM isoprenaline was decreased significantly in the presence of phenoxybenzamine (100 muM). In guinea-pig atria, phenoxybenzamine had no effect on myocardial fluorescence. Fluorescence intensity was also decreased if the incubation with isoprenaline was carried out at 0 degrees C or if the post-incubation washing temperature was 37 degrees C instead of 0 to 2 degrees C.6 The results demonstrate that the fluorescence histochemical technique can be used to locate isoprenaline in tissues. They also indicate that guinea-pig and cat trachealis smooth muscle cells and cat atrial myocardial cells can accumulate isoprenaline (a) by a mechanism sensitive to phenoxybenzamine and (b) into sites in which COMT plays a functional role in inactivating isoprenaline at the concentration used in these histochemical experiments (50 muM). In contrast, the guinea-pig atrial myocardial cells may have a minimal capacity to accumulate isoprenaline by a phenoxybenzamine-sensitive uptake mechanism.