The serum decay and tissue distribution of iodine-labeled high density lipoprotein (HDL)-apoproteins were measured in rats 2--8 h after partial hepatectomy or sham-operation. A method was developed allowing continuous bloodsampling without using anticoagulantia or anaesthetics. The serum decay of HDL-apoproteins was biexponential. Neither the initial rapid phase (t 1/2 0.3 +/- 0.1 h), nor the slow phase (t 1/2 6.2 +/- 0.3 h) were influenced by the removal of 2/3 of the liver and consequently there was no effect on the fractional catabolic rate (F.C.R.: 2.9 +/- 0.2/day). The level of circulating HDL was decreased by partial hepatectomy but the chemical composition of HDL was unchanged. Total tissue HDL radioactivity in the control rats was 5.7, 2.8, 2.7, 1.0, 0.7, 0.2, 0.4 and 0.1% of the injected dose for skeletal muscle, adipose tissue, liver, jejunum, kidneys, spleen, lungs and heart, respectively. Only the value for liver was affected significantly by partial hepatectomy (0.6%). It is concluded that the in vivo degradation rate of HDL-apoproteins is not influenced by the removal of 2/3 of the liver and that the decrease in serum HDL concentration is due to an impaired rate of hepatic synthesis. These results indicate the possiblity of extrahepatic HDL-apoprotein catabolism or a stimulation of HDL-apoprotein degradation, induced by partial hepatectomy, in the remaining liver lobes.