The present study examined whether alpha-adrenergic stimulation causes a change in splanchnic intravascular volume in the anaesthetized animal with an intact circulation, which region(s) mediate the volume change, and whether the splanchnic volume change influences cardiac output. In order to ascertain that a radionuclide imaging technique could be used to assess total splanchnic volume changes, drugs known to increase or decrease splanchnic volume were infused on 21 occasions in eight dogs studied under conditions of selective perfusion and drainage of the splanchnic vasculature with erythrocytes labelled with 99Tcm and a gamma camera placed over the abdomen. For these 21 infusions, volume and radionuclide count changes were related: r greater than or equal to 0.90 (n = 20), r = 0.76 (n = 1). After ascertaining tissue attenuation and blood radioactivity in four of the animals, the standard error for a single estimate of the absolute volume change using the radionuclide technique was determined to be 75 ml. In six animals with intact circulations, phenylephrine (40-80 micrograms min-1) for 20 min was associated with an increase in cardiac output of 12 +/- 2% (P less than 0.001) and a decrease in total splanchnic volume estimated to be 431 +/- 95 ml (P less than 0.001). The splanchnic volume decrease was due entirely to decreases in splenic and intestinal volume. In eight eviscerated animals, cardiac output decreased by 30 +/- 2% (P less than 0.001) during phenylephrine.(ABSTRACT TRUNCATED AT 250 WORDS)