1. The histamine receptors were characterized on isolated circular segments of trachea and pulmonary arteries from the guinea-pig. The motor responses to histamine H1-, H2- and H3-receptor agonists and antagonists were tested and the responses obtained were analysed in relation to the respiratory epithelium and the vascular endothelium. 2. Histamine induced a biphasic response in trachea and in pulmonary arteries. In low concentrations, histamine acted as a potent relaxant agent of precontracted segments and in moderate concentrations it constricted both precontracted and resting segments. When arterial segments from different parts of the pulmonary vascular tree were compared, only small interregional differences in the vasomotor response were seen. 3. Mepyramine caused a parallel shift to the right of the histamine-induced concentration-response curves for both the trachea and the pulmonary artery, indicating a contractile H1-receptor. Cimetidine did not affect the histamine-induced contraction of the trachea, but a shift to the left was evident for low concentrations of histamine in the pulmonary artery. This is consistent with a dilator H2-receptor in the pulmonary artery. The pA2-value for mepyramine in the pulmonary artery, 8.75, was not affected by the presence of cimetidine. Thioperamide, a selective H3 antagonist, shifted the concentration-response for the trachea to the left. Schild analysis for histamine and mepyramine yielded a line with a slope of 0.61, whereas the same analysis in the presence of thioperamide yielded a line with a slope of 1.05 and an approximated pA2-value of 9.57. These results indicate the presence of a relaxant H3 receptor in the trachea. In precontracted tracheal segments, application of mepyramine and cimetidine did not affect the low dose histamine relaxation. Thioperamide caused a parallel shift of the histamine concentration-response curve to the right, supporting the suggestion of a dilator H3-receptor in the trachea. The pA2-value for thioperamide, in the presence of mepyramine, was 7.79. In precontracted pulmonary arteries the histamine-induced dilatation was small. In the presence of mepyramine a rather strong histamine-induced dilatation became evident and this concentration-response curve could be shifted to the right by cimetidine, with a pA2-value of 6.49. This is compatible with a dilator H2-receptor. 4. The H1-receptor agonists, thiazolylethylamine, 2-methylhistamine and pyridylethylamine and the rather unselective H2-agonist, 4-methylhistamine, induced contraction of resting tracheal and pulmonary arterial segments. In precontracted segments of trachea, all H1 and H2 agonists studied induced a dilator response. The two rather unselective histamine receptor agonists 2-methylhistamine and 4-methylhistamine were about 100 times more potent than other H1 and H2 agonists tested. In the pulmonary artery, the H2 agonists, impromidine, dimaprit and 4-methylhistamine induced a concentration-dependent relaxation. The relaxation of the pulmonary artery, elicited by the H1 agonists,thiazolylethylamine and pyridylethylamine, was smaller, but more potent than the response induced by the H2 agonists. This may reflect the presence of a separate dilator H1-receptor.5. R-alpha-methylhistamine induced a three phased response in precontracted tracheal segments. In low concentrations, a concentration-dependent dilator response appeared. At moderate concentrations, a stage with a plateau or a small contraction was seen, followed at high concentrations by a new concentration-dependent relaxation. The first dilator phase was similar to that obtained for histamine in the same preparation.6. Removal of the epithelium or endothelium enhanced the contractile histamine response in both the trachea and the pulmonary artery as well as the dilator response in the trachea. These results support the hypothesis that the endothelial layer may serve as a barrier against the penetration of certain mediators. In the precontracted pulmonary artery, the small initial dilatation was abolished whereas the second dilatation seen in the presence of mepyramine was slightly reduced. This may reflect the influence of two separate histamine receptors, one of which is associated with the release of an endothelium dependent dilator factor or factors.7. In the guinea-pig trachea, histamine-induced contraction is mediated through H1-receptors where as dilatation probably involves an H3-receptor on the smooth muscle. The guinea-pig pulmonary artery appears to be endowed with a contractile H1 receptor on the smooth muscle cells and a dilator H1 receptor located on the endothelium. A dilator H2 receptor on the smooth muscle cells seems, at least in vitro, to be the most potent mediator of histamine-mediated pulmonary arterial dilatation.