Triacylglycerols were the major lipid class in the fatty livers from alloxan-diabetic sheep and those suffering from pregnancy toxaemia, with the concentrations increased by 15- and 25-fold, respectively, compared with the normal state. Analysis of the fatty acid composition of total liver triacylglycerols in these animals showed a significant decrease in the proportion of saturated fatty acids, 16:0 and 18:0, and increase in the proportion of polyunsaturated fatty acids (18:2 omega 6, 18:3 omega 3 and 20:4 omega 6), particularly in those with pregnancy toxaemia. In contrast, total liver phospholipids showed a significant increase in the proportion of 18:0 in ewes with pregnancy toxaemia and a significant decrease in a range of polyunsaturated fatty acids in both the diabetic and toxaemic animals. Also, although the concentration of both phosphatidylcholine and phosphatidylethanolamine increased in the diabetic livers the ratio of phosphatidylcholine/phosphatidylethanolamine fell significantly, from 2.22 in the control animals to 1.59. The data suggest that, following the large influx of plasma fatty acids into the ovine liver in diabetes and pregnancy toxaemia, there is a diversion of polyunsaturated fatty acids from phospholipids to triacylglycerols. In diabetic sheep these changes may in turn affect phosphatidylcholine synthesis via the methylation pathway in liver. These changes in lipid composition may, in part, explain the degenerative changes in membrane and subcellular organelle structure and the failure of liver function observed both in advanced diabetes and in severe pregnancy toxaemia.