The fatty acid composition of the plasma choline phosphoglycerides (CPG) and erythrocyte ethanolamine phosphoglycerides (EPG) were detemined in a series of patients suffering from endogenous depression, a number of patients from the same ward with non-depressive psychiatric disorders, and a number of age- and sex-matched healthy control subjects. A significantly higher proportion of long chain polyunsaturated fatty acids, in particular eicosapentaenoic acid (20:5omega3) and docosahexaenoic acid (22:6omega3), was found in the subjects with endogenous depression compared with the controls. This change was reflected to a lesser extent in the erythrocyte EPG. The proportions of the two prostaglandin precursors gamma-dihomolinolenic acid (20:3omega6) and arachidonic acid (20:4omega6) were similar in both groups. The fatty acid composition of the plasma CPG and erythrocyte EPG of a number of subjects from the same ward with non-depressive psychiatric disorders did not differ from that of the healthy controls. It is suggested that the analysis of the fatty acid composition of plasma CPG may be of diagnostic value and could be involved in the pathology of endogenous depression.