The undecapeptide substance P (SP) is both a potent stimulant of intestinal (guinea pig ileum), vascular venous (rabbit mesenteric vein) and tracheal (guinea pig trachea) smooth muscles, and a relaxant of large arteries (dog common carotid artery, rabbit aorta). SP also decreases the peripheral vascular resistance in the coronary vessels of rabbit isolated hearts. The contractions of the guinea pig ileum in response to SP are partially antagonised by atropine and significantly depressed by indomethacin, while the contractions of the guinea pig trachea are strongly potentiated by indomethacin. Large arteries, but not the veins, are insensitive to SP when the endothelium is removed, and slightly less responsive to the peptide when this is applied in the presence of mepacrine, eicosatetraynoid acid or BW 755C. These results indicate that all myotropic actions of SP, except that on venous smooth muscle, are partially or completely indirect and most of them are due to the release of endogenous agents: these are acetylcholine, prostaglandins and possibly histamine in the guinea pig ileum, prostaglandins and possibly a leukotriene in the guinea pig trachea and a still unidentified factor of endothelial origin in dog and rabbit large arteries. The effect of substance P on the rabbit isolated anterior mesenteric vein appears to be a direct one.