Responses to changes in intravascular pressure of isolated rat gracilis muscle arterioles were investigated under no-flow conditions. First-, second-, and third- order arterioles were isolated and cannulated. Vascular diameters were measured with an image-shearing device and then recorded. In response to the step increases in perfusion pressure (from 20 to 160 mmHg, by 10- or 20-mmHg steps) arterioles constricted and developed active tone. For example, at 100, 80, and 50 mmHg pressure the steady-state active diameters of 1st-, 2nd-, and 3rd-order arterioles were 76.9 +/- 1.6, 32.3 +/- 1.1 and 22.3 +/- 3.2 microns, respectively. At the same perfusion pressure, by use of a Ca(2+)-free solution (ethylene glycol-bis(beta-aminoethyl ether)-N,N,N',N'-tetraacetic acid; 1 mM) containing sodium nitroprusside (SNP; 10(-4) M), the passive diameters (PD) of these vessels were 161.8 +/- 3.2, 76.0 +/- 1.7, and 47.6 +/- 2.2 microns. The negative slopes of the pressure-diameter curves indicate that in the physiological pressure range an inverse relationship exists between the arteriolar diameter and intravascular pressure. The maximum constriction expressed as a percent of PD was similar in the various sized arterioles (approximately 60%) but was reached at lower pressures in the smaller vessels. The vasoactive function of endothelium and vascular smooth muscle was assessed by the responses of arterioles to acetylcholine (ACh; 10(-6) M) and SNP (5 x 10(-8) M) before and after removal of the endothelium with air. After removal of the endothelium, dilation to ACh was abolished while dilation to SNP was retained.(ABSTRACT TRUNCATED AT 250 WORDS)