A method of examining arteriolar function in situ using the in vitro-perfused hydronephrotic rat kidney is described. This approach facilitates direct visualization of arteriolar contractile responses in a well-controlled experimental environment while avoiding the consequences of traumatic microdissection and the accompanying exposure to ischemia, hypothermia, or hypoxia. The preparation has a remarkably well-preserved myogenic reactivity, exhibiting precisely graded vasoconstriction over the range in perfusion pressure subtending normal renal autoregulatory responses (i.e., 80-180 mm Hg). Using this preparation, the inhibitory effects of hypoxia on arteriolar myogenic reactivity have been demonstrated. The range over which reduced pO2 affected arteriolar reactivity in this model corresponded closely to that reported to alter vascular tone in vivo. A technique of adapting the model to incorporate simultaneous monitoring of arteriolar fluorescence measurements and contractile responses is also described. This approach has been used to examine the relationship between arteriolar contractility and NADH autofluorescence during the hypoxia-induced activation of ATP-sensitive K channels. Future applications may include the use of intravital fluorescent dyes to examine, for example, microvascular endothelial calcium signaling in an intact, functioning arteriole.