An in vitro constant-flow, alternatively constant-pressure, perfusion method for studying microvascular myogenic responsiveness in otherwise intact rat hindquarters was used to explore how three types of influences affect the pressure-flow induced enhancements of myogenic tone: (1) Changes of potassium concentration over the range 2-50 mmol; (2) Whether myogenic responsiveness, which is fairly weak during these conditions of high shear rates, is enhanced by blockade of endothelial prostacyclin and nitrous oxide formation; (3) Whether some of the drawbacks of the constant-flow technique, like rapid edema formation, is offset by a constant-pressure variant. The result can be summed up as follows. (1) Myogenic responsiveness is also during in vitro conditions markedly influenced by even modest potassium concentration changes around the physiological resting level, e.g. showing a fourfold difference between potassium concentrations of 2.5-4.5 mmol. It was entirely suppressed above 7-8 mmol potassium concentration but also when potassium concentration values above 20 mmol increasingly depolarize and constrict the resistance vessels. (2) While blockade of prostacyclin synthesis was without effect, suppression of endothelial nitrous oxide production could increase myogenic responsiveness up to fourfold, suggesting that shear-stress dependent release of this agent serves to suppress myogenic activity. These effects were, however, quite variable suggesting that also other endothelial inhibitory influences are involved. (3) The constant-pressure variant reduces some of the methodological drawbacks, particularly the oedema formation which can be further reduced by increasing perfusate colloid osmotic pressure. However, interferences by the mentioned, partly unknown endothelial mechanisms still tend to suppress myogenic responsiveness to a varying extent, which hampers quantitative analyses particularly of other inhibitory influences.