Changes in arterial plasma levels of atrial natriuretic factor and catecholamine release were studied in 11 beagle dogs during pentobarbital anesthesia. Seven dogs were injected intravenously with Escherichia coli endotoxin, 0.5 mg/kg over 15 min. Four control dogs received only saline solution. The endotoxin injection resulted in cardiac depression,, hemoconcentration, acidosis and renal hypoperfusion. The central venous pressure remained relatively unchanged in both groups during the 2-hour study. The concentrations of epinephrine, norepinephrine and the norepinephrine metabolite 3,4-dihydroxyphenylglycol increased in arterial plasma during the acute hypodynamic endotoxin shock. In the control dogs the levels of these hormones remained very low and constant. Increased circulating levels of atrial natriuretic factor were observed in endotoxin shock with renal hypoperfusion, unchanged central venous pressure and no concomitant tachycardia.