Changes in lipidaemia and in cholesterol esterification rate were investigated in 65 patients with chronic mesangial glomerulonephritis (GN), and in 26 patients with polycystic kidneys (PL), as well as in age- and sex-matched control groups. As compared to the controls, a slightly significant increase in cholesterol and triglyceride concentrations was found only for the GN group, whereas the rate of cholesterol esterification showed a highly significant reduction in both groups of diseased subjects. The average values of molar esterification rate (MER) were, respectively, 75.4 and 61.6 mumol . l-1 . h-1 for the GN and PL groups, the respective control values being 96.9 and 91.1. Fractional esterification rate (FER) reflecting the rate of cholesterol exchange between blood and tissues fell in the same two groups of patients to 4.38 and 4.40% . h-1, respectively (controls 6.93 and 6.17). Both the changes in cholesterol esterification rates and a relative increase in the ratio of unesterified to esterified cholesterol were found in patients with low as well as normal glomerular filtration rates.