The entry and movement of norepinephrine through the media of the isolated rabbit thoracic aorta has been investigated using an isotopic frozen section technique. After 1-minute exposure of the intimal surface of a helical strip to Krebs bicarbonate solution containing tritiated norepinephrine (3H-NE), the 3H material was distributed throughout the thickness of the wall. The highest concentration occurred at the innermost layer of the media and gradually decreased toward the adventitial side. The shape of the tritium distribution curves was not significantly changed by previous treatment of aortic strips with either cocaine, phenoxybenzamine or the MAO and COMT inhibitors, pargyline and tropolone, respectively. Temperature reduction to 27°C had no significant effect on the tritium distribution curves. The apparent diffusion coefficient for 3H-NE calculated for movement into the extracellular (inulin) space of the media was 7.29 x 10-7 cm2/sec. This relatively slow rate of diffusion is probably due to physical barriers such as elastic lamellae within the tunica media.