Possible correlations between lipid metabolism and hydroelectrolytic and acid base balances were studied in pathological conditions. For this purpose 24 patients with essential dyslipidaemia without renal or hepatic disease, and 47 patients with varying disease conditions, 25 with profound alterations in the lipid balance, 22 with plasmatic lipid fractions completely normal, were studied. In cases of essential hyperlipidaemia, alongside the profound changes in the lipid complement, no significant alterations in plasma pH were observed; by contrast other cases frequently showed an upset acid base balance with or without changes in the lipid picture. Urinary determinations suggested different conclusions. The most important aspect was the observation in most cases of essential dyslipidaemia and in certain cases of secondary dyslipaemia, of paradoxical aciduria, namely aciduria of much greater extent than might have been expected on the basis of plasma pH value.