The binding of labeled norepinephrine to brown adipose tissue intact microsomes or solubilized microsomal proteins was studied in vitro. The effects of incubation in oxygenated Krebs-Ringer bicarbonate buffer, pH 7.4, on the physicochemical state of norepinephrine was investigated. After 10 minutes of incubation, the recovery of norepinephrine (0.5 μM) by alumina adsorption technique was found to be only about 40 per cent and no oxidation was detected by polarography. The recovery could be increased to over 90 per cent by adding metal chelators to the Krebs-Ringer bicarbonate buffer or by incubation in a phosphate buffer, which suggests that contaminating metals can cause considerable complexation of the hormone. The assays with unmodified norepinephrine were therefore performed under the following conditions: 10 min incubation in 50 mM phosphate buffer, pH 7.4. 25°. In both intact microsomes and solubilized microsmal proteins, the binding of norephinephrine was found to be sensitive to substances affecting catechol- O-methyl transferase activity such as tropolone, normeta nephrine, dithiothreitol, Ca 2+ and Ca 2+ chelators; agents that inhibited catechol- O-methly transferase activity were shown to stimulate norepinephrine binding and vice versa. After separation of solubilized microsomal proteins by Ultrogel AcA 34 filtration, both norepinephrine binding and catechol- O-methly transferase activity were found in the same protein fraction. Separation by polyacrylamide gel electrophoresis revealed congruent migration of norepinephrine binding, catechol- O-methyl transferase activity and S-adenosyl methionine binding.