The effects of hemorrhages on plasma flow velocity and indicator (FITC[fluorescein isothiocyanate]-Dextran) dilution in series and parallel-coupled microvascular vessels were studied. The mesentery (0.6 kg) of cats anesthetized with Dial-Urethane was exposed and studied by fluorescence microscopy. Indicator was injected as a bolus (0.1 ml/0.5 s) in a small branch of the mesenteric artery. Indicator dilution curves were videotape recorded by a video sampler with intensity sensitive windows. The mean transit time (t), appearance time (ta), peak time (tp), curve duration (tE), plasma velocity and vessel diameter changes were determined. Arterioles 68 .+-. 8 .mu.m and 35 .+-. 3 .mu.m and venules 55 .+-. 6 .mu.m constricted gradually with hemorrhage. Arterioles 20 .+-. 2 .mu.m constricted with mild hemorrhage and remained constricted with further hemorrhage. Arteriolar and venular plasma velocities decreased abruptly with mild hemorrhage and then continued to decrease gradually with further hemorrhage. Capillary flow ceased after moderate hemorrhage and was not reestablished until at least 1 h after reinfusion. Arteriols t increased with hemorrhage but t increased much more in capillaries and venules. tE changes paralleled t changes. The ratio ta/t was lower for venules than for arterioles and both decreased with hemorrhage but the venular ratio decreased significantly more. Venular tp-ta and tE-tp increased more than arteriolar values. Hemorrhage increased indicator dilution with the greatest effect occurring between the arteriolar and venular vessels.