The effects of thiopental sodium on the adrenergic neuroeffector junction were studied in isolated rabbit pulmonary arteries. Basal tension was not altered by thiopental (2 X 10-5and 10-4M) but was increased by high concentrations of thiopental (5 X 10-4M). Thiopental (10-4and 5 X 10-4M) potentiated contractions induced by transmural electrical stimulation. Contractile responses to exogenously applied low concentrations of norepinephrine (NE) were potentiated by thiopental (2 X 10-5, 10-4and 5 X 10-4M), whereas those to high concentrations were not altered. In strips previously incubated in 1-[7,8-3H]-NE (10-7M), the release of [3H] induced by transmural stimulation (5 Hz) was not altered by thiopental (10-4and 5 X 10-4M). Potentiation by thiopental (10-4M) of the responses to transmural stimulation was not affected by prior application of cocaine or hydrocortisone. Contractions induced by alpha receptor agonists (phenylephrine and methoxamine) were potentiated by thiopental (10-4M), while those induced by acetylcholine were not altered. Contractile responses to potassium chloride were attenuated by thiopental (10-4M). Amobarbital sodium and pentobarbital sodium (10-4M, respectively) attenuated contractions induced by NE. It may be concluded that thiopental specifically increases the responsiveness of postsynaptic alpha receptors to NE.