The effects of phenoxybenzamine and norepinephrine (NE) on the release of tritiated NE were studied in spiral strips of the rabbit pulmonary artery. Phenoxybenzamine increased the tritiated NE efflux induced by transmural stimulation at 4 Hz by a factor of three. The magnitude of this effect was inversely related to the frequency of stimulation. NE (2.5 × 10–6 m) decreased the enhancing effect of short exposure but not long exposure to phenoxybenzamine, probably by surmounting the phenoxybenzamine blockade of the α-adrenergic receptors on the adrenergic nerve terminal. The transmitter output per pulse increased with the frequency from 4 to 16 Hz and decreased at 32 Hz. This evidence suggests that a negative feedback mechanism does operate in this vascular preparation but, probably because of the wide neuromuscular cleft, is less effective than in tissues with a narrower synaptic interval.