[3H]norepinephrine binds in vitro to microsomal membranes derived from a wide variety of tissues. Controversy exists as to the physiological significance of this binding phenomenon and recently the suggestion has been made that this represents binding to the enzyme catechol-o-methyl transferase (COMT). The enzyme and the [3H]norepinephrine binding sites however, are shown to have very different characteristics. Tissues having the highest levels of microsomal catechol-o-methyl transferase(liver, kidney) bind the least [3H]norepinephrine. The enzyme is concentrated in soluble fractions of tissue whereas the binding sites are found almost exclusively in membrane fractions. Purified preparations of the binding sites have no catechol-o-methyl transferase activity. pH optima, KM for epinephrine, and substrate specificity are all strikingly different for the enzyme and the [3H]norepinephrine binding sites. It is concluded that in vitro[3H]norepinephrine binding is unrelated to microsomal catechol-o-methyl transferase. © 1974 Academic Press, Inc.