1. The present study was designed to investigate a possible role of thromboxane A2 (TXA2) on arteriolar vasomotion (spontaneous rhythmic variations of the vessel diameter). Therefore the microcirculatory effects of the thromboxane-receptor (TP-receptor) agonist, U 46619, as well as the effects of the TP-receptor antagonists S 17733 and Bay U3405 were evaluated in the hamster cheek pouch microcirculation. For comparison some effects of angiotensin II were also investigated. 2. For microcirculatory measurements, the cheek pouch preparation was placed under an intravital microscope coupled to a closed circuit TV system. The TV monitor display was used to obtain arteriolar internal diameter measurements by means of an image shearing device. 3. Superfusion (0.1 nM to 1 microM) or bolus application (1 pmol to 10 nmol) of U 46619 concentration- or dose-dependently decreased the arteriolar diameter and induced vasomotion in arterioles with a mean initial diameter of 24+/-2 microm. Both the vasoconstriction and the vasomotion induced by U 46619 were inhibited by the TP-receptor antagonists S 17733 (100 mg kg(-1), i.v.) and Bay U3405 (10 mg kg(-1), i.v.). 4. Bolus applications of angiotensin II (0.1 pmol to 1 nmol) induced transient vasoconstriction followed by vasodilatation in the cheek pouch arterioles. The dilatation but not the constriction, was sensitive to treatment with the NO-synthase inhibitor N(omega)-nitro-L-arginine (L-NOARG; 100 microM). Angiotensin II did not induce vasomotion in control conditions or in the presence of L-NOARG. 5. Bolus application of phenylephrine (10 pmol) induced vasoconstriction but no vasomotion in previously quiescent hamster cheek pouch arterioles. 6. These results indicate that activation of TP-receptors causes vasomotion in the hamster cheek pouch arterioles. These spontaneous rhythmic variations in arteriolar diameter are not observed with equipotent doses of angiotensin II and phenylephrine. Thus, the vasoconstriction by itself cannot explain the occurrence of vasomotion observed with the TP-receptor agonist.