The stability of plasma lipids and apolipoproteins during the early postmortem period was studied by taking four duplicate blood samples from eight cadavers 2, 6, 12, and 24 h after death. The bodies were kept at +4 degrees C. The plasma samples were analyzed for total cholesterol (TC), triglycerides (TG), apolipoprotein B (apo B), and apolipoprotein A-I (apo A-I). In TC, values rose by 6 and 11% in two cases, and in six cases diminished 3 to 15% during the first 6 h compared to values obtained 2 h postmortem. The greatest changes were a continuing rise in one case by 33% and a fall by 21% in another case during 24 h. In TG values marked changes took place including one case with a rise of 67% within 24 h. The concentrations of apo B rose by 9 to 11% in three cases and fell by 3 and 4% in two cases during 6 h, but during the whole study period a rise up to 78% occurred. In the concentrations of apo A-I, cases fell by as much as 42% in 6 h, and in one case rose by 20% during 6 h. The results indicate that unpredictable fluctuations occur in plasma lipid and apolipoprotein values within 24 h after death, and they should be interpreted cautiously if the samples have been taken after a prolonged postmortem period.